compiler: implement blank package name semantics

docs/test-system-v2.md

@@ -339,6 +339,146 @@ Build workdir format remains 18, test workdir format remains 19, and semantic
 storage format remains 3; no test-result cache, schema, action descriptor,
 transaction marker, or lock is introduced.
 
+The blank declared package name has a different contract from documentation
+suppression. The sole authority is official Go 1.26.5 at commit
+`c19862e5f8415b4f24b189d065ed739517c548ba`:
+
+- **behavior directly implemented or asserted by pinned Go** — the compiler
+  scanner admits and dispatches `_` through its name path
+  (`cmd/compile/internal/syntax/scanner.go:88–107,368–394,437–439`), the parser
+  accepts it in package syntax (`parser.go:397–420,2751–2763`), and types2
+  rejects the retained node as `invalid package name _`
+  (`cmd/compile/internal/types2/check.go:336–355`).
+- **behavior directly implemented or asserted by pinned Go** — official
+  `test/blank1.go:1–31` asserts the blank-name error and later checker errors,
+  proving continuation. `internal/types/testdata/check/blank.go:1–5` asserts
+  only the blank-name error. `cmd/go/testdata/script/build_test_only.txt:1–18`
+  and `build_no_go.txt:1–30` are surrounding source-role controls.
+- The official tree contains no cmd/go blank-name matrix for named, multiple,
+  mixed, imported, test-only, action, artifact, rollback, concurrency, or
+  interruption cases; the focused WW package observer owns those proofs.
+- **behavior directly implemented or asserted by pinned Go** — types2
+  validates an importer-supplied package object before applying its local
+  alias. Provider name `_` emits
+  `could not import PATH (invalid package name: "_")` at the source import
+  path, caches a path-leaf-named fake package, marks the use, and continues
+  (`cmd/compile/internal/types2/resolver.go:125–180,248–335`; public twin
+  `go/types/resolver.go:157–190,263–350`). An empty provider name quotes the
+  actual empty value instead.
+- No official pinned test directly supplies an imported `Package` named `_`.
+  **behavior derived from the pinned implementation** — the
+  imported-interface rows are WW-native proof of applying those resolver
+  semantics to WW's supported source-like interface channel.
+
+The slice is not owned only by the four primary parser/checker files. The two
+`w6c` command fronts own test-support materialization, imported-package
+metadata, reachability/filtering, fake qualifier assignment, and delayed AST
+concatenation; the checker twins own deduplicated diagnostics and empty-scope
+fake recovery; the AST/parser twins own the independent path-token position.
+The package coordinator and public drivers exercise the same interface channel
+without redefining package or action identity.
+
+The coordinator treats `_` as a syntactically loaded declared name, not a
+generic missing-clause error. A production-only blank package under `ww test`
+selects its one ordinary production compiler action and fails there. Blank
+production plus a same-package blank `_test.ww` selects one augmented internal
+test action containing both source sets. `sep_recompile_for_test` substitutes
+that action for the separate production node throughout the product closure,
+so it is compiled once and emits one `invalid package name _` per retained
+source marker in deterministic unit order. A test-only blank `_test.ww` forms
+one test-only internal action and reports once. Valid production `p` plus blank
+test `_`, or blank production `_` plus an unrelated `p` or `p_test`, keeps the
+existing family-mismatch rejection before tools. Blank production plus the
+actually related external name `__test` passes family classification, but the
+production dependency fails before the external action, support/generated
+main, link, or runtime completes.
+
+The observable test phase order is selected-source validation and package/test
+family classification, recursive import loading, eligible dependency and
+support producers, complete parent-source parsing, blank-name checking,
+imported-package validation, later checker diagnostics, and the
+existing package failure trailer. A missing or invalid dependency may
+therefore suppress both a later body syntax diagnostic and BlankPkgName; a
+complete-file syntax error suppresses checker diagnostics.
+Raw and directory running requests keep their established command-owned
+`FAIL\n` placement, while `-c` and applicable `-S` paths retain empty stdout.
+Directory `-S` option-shape errors remain earlier than source selection. No
+blank product begins a test or user runtime.
+
+The imported-interface observer uses the supported repeatable
+`w6c -c --import PATH FILE.wwi` channel and its public persistent build/test
+equivalent. Each compiler first read/owner/syntax-checks sorted standalone
+interfaces, then parses primary source. Primary syntax failure precedes
+imported-package semantic validation. After successful primary syntax, `-T`
+materializes a missing compiler-required `test` or collision-safe `__wwtest`
+use in the primary list before resolution. It is marked used and rooted at the
+generated primary file position. With no matching import it keeps the existing
+external-support fallback; with a matching blank-named interface it receives
+the ordinary invalid-provider diagnostic. Existing explicit test-target roots
+already have a primary occurrence and are not duplicated.
+
+The observer requires metadata classification as valid, missing, conflicting,
+or invalid, followed by reachability seeded solely by canonical primary and
+compiler-required uses. Only a reached valid interface owner may contribute
+transitive imports. Before binding and delayed concatenation, invalid-owner,
+unreachable, and ownerless standalone facts are removed. The unused-interface
+control includes a mixed-origin invalid interface whose embedded valid origin
+imports the bad path: because that origin is not independently rooted, the
+whole interface is inert and the primary outputs equal the no-interface
+control. A companion primary-reached origin proves that legitimate traversal
+does reach and diagnose the bad provider.
+
+For every retained bad use, default, explicit, and blank aliases all validate
+the provider name. Nonblank uses receive an explicit-alias or canonical-path-
+leaf fake binding with empty scope; blank uses install none. All are marked
+used. Qualified value/call/type recovery produces no missing-member,
+unknown-type, export, or calling-nonfunction cascade, while lexical shadowing
+still applies. One canonical bad path emits one diagnostic despite repeated
+occurrences; two paths emit one each in retained order, and an independent
+checker diagnostic follows. Every source-created form points at the path's
+first identifier, not the explicit alias. The compiler-generated support use
+uses its generated root position because no path token exists.
+
+Public proof corrupts a caller-owned committed provider `.wwi`, then exercises
+build and test consumers through the existing canonical interface action.
+Reached corruption fails the affected compiler action before its assembler and
+downstream archive, link, retention, or runtime. A package-action failure
+prevents generated main; a generated-main action that is itself the consumer
+performs the same validation before its assembly. A same-named `.ww` file is
+still an import decoy. Raw, production, internal, external, test-only,
+generated-main, `-c`, and applicable `-S` shapes share the same validation.
+Prior public, retained, and semantic bytes survive, invalid bytes cannot commit
+a replacement consumer, and restoring valid interface bytes follows ordinary
+reuse. An unreferenced corrupt interface remains inert and causes no
+invalidation.
+
+The package-name token does not alter production/internal/external/recompiled,
+support, or generated-main identities. Direct actions retain `__root`, dotted
+actions retain dotted identity, and no successful ordinary producer interface
+can advertise a blank provider. A supplied or corrupted interface that does is
+invalid consumer input. A failed primary blank action or reached invalid
+interface consumer emits no assembly or interface, so its assembler, archiver,
+linker, generated main, harness, and runtime do not run; earlier valid
+dependency/support producers may run normally. Request rollback removes owned
+stages and commits no failed unit, interface, assembly, object, archive, binary,
+status, retained output, or tool stamp. Warm prior public and semantic bytes
+remain unchanged, exact source/interface restoration uses ordinary reuse, and
+concurrent requests keep parser/checker reachability, deduplication, fake
+bindings, and cleanup independent.
+
+Direct `w6c`/`w6c_ww` and public `ww`/`ww_ww` are required to agree on status,
+stdout, exact path-token positions and diagnostic order, fake recovery, output
+absence, and prior-byte preservation for the full role/import matrix. The
+syntax/check change adds no test-process topology, test-result cache, schema,
+action descriptor, or stored identity. Private in-memory `N_USE` nodes gain
+only path-position fields; AST enum/printing and `.wwi` serialization are
+unchanged. Build workdir format remains 18, test workdir format remains 19, and
+semantic storage format remains 3. Normal checker failure leaves no anonymous
+descriptor, adjacent stage, request residue, or child. External-driver SIGTERM
+supervision is unchanged: the known orphan compiler, fixed `.new` staging, and
+later persistent-request poisoning remain open and are not credited to this
+slice.
+
 An existing local directory whose requested build basename ends `.ww`
 (including a visible `_test.ww` symlink to a directory) remains a directory
 package, not a raw named test source. WWstage `ww build` now uses the same
@@ -1144,8 +1284,15 @@ timeout policy in this architecture.
 
 ## Open driver work
 
-None; directory-package `-c` and `-o` now have the applicable Go 1.26.5
-retention, naming, fan-out, execution, and publication behavior.
+Directory-package `-c` and `-o` have the applicable Go 1.26.5 retention,
+naming, fan-out, execution, and publication behavior. Direct external SIGTERM
+of either driver during blocked persistent compilation remains open: prior
+public and committed semantic bytes survive, but the directly spawned compiler
+can survive with three fixed-name `.new` stages, and a later persistent request
+then rejects the existing `.unit.new`. The required repair is driver-owned
+child-process-group supervision and normal request rollback before re-raising
+the original signal; blind removal of `.new` files cannot distinguish foreign
+or concurrent stages.
 
 ## Validation policy