ww build: name current directory output

docs/build-system.md

@@ -8119,6 +8119,133 @@ This is runtime/coordinator presentation, not a persisted-byte contract. Build
 workdir format remains `18`, test workdir format remains `19`, and semantic
 storage format remains `3`.
 
+### 11.40 Implemented current-directory default build names
+
+An empty `ww build` package list selects the current directory just as an
+explicit `.`, `./`, or equivalent sequence of single-dot components does.  For
+one command package and no explicit `-o`, the public executable is now named by
+the selected directory's final component.  The selector is loading syntax; it
+is not the literal output pathname.  Thus a command built while the current
+directory is `tool` publishes `tool`, not `.`, and cold private build artifacts
+use `tool.sepwork`, not `..sepwork`.
+
+A non-main current-directory package uses the same corrected cold scratch stem,
+but still has no link or public installation action.  An explicit `-o`, exact
+`-o /dev/null`, an ordinary non-current directory operand, and a contextual
+dotted package request keep their established output rules.
+
+#### Pinned Go evidence and fact classification
+
+The sole semantic authority is official Go 1.26.5 at commit
+`c19862e5f8415b4f24b189d065ed739517c548ba`:
+
+- `search.CleanPatterns` turns an empty package-pattern list into exactly `.`;
+  `ImportPathsQuiet` then treats that local pattern as a directory selection
+  ([`cmd/go/internal/search/search.go`, lines 441–480](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/search/search.go#L441-L480)).
+- `work.runBuild` loads packages and checks load errors before output planning.
+  With exactly one loaded `main` package and no `-o`, it selects
+  `DefaultExecName`
+  ([`cmd/go/internal/work/build.go`, lines 459–478](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/work/build.go#L459-L478)).
+- `load.(*Package).exeFromImportPath` takes the final loaded import-path
+  element, while `DefaultExecName` uses a source basename only for a
+  command-line-files package
+  ([`cmd/go/internal/load/pkg.go`, lines 1727–1769](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/internal/load/pkg.go#L1727-L1769)).
+- Official command testdata distinguishes module naming from the manifest-free
+  GOPATH case and requires bare `go build` to create the directory-named `src`
+  executable
+  ([`clean_binary.txt`, lines 15–28](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/clean_binary.txt#L15-L28)).
+  A second manifest-free script changes to `m`, runs bare `go build`, and
+  requires executable `m`
+  ([`gccgo_m.txt`, lines 4–14](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/gccgo_m.txt#L4-L14));
+  `build_static.txt` likewise builds and executes the default `hello`
+  ([lines 11–14](https://github.com/golang/go/blob/c19862e5f8415b4f24b189d065ed739517c548ba/src/cmd/go/testdata/script/build_static.txt#L11-L14)).
+
+Those selection, loading, default-name branches and script assertions are
+**behavior directly implemented or asserted by pinned Go**.  WW has no module
+or manifest identity for a literal root, so using the selected local directory
+leaf as its already-specified presentation fallback is **behavior derived from
+the pinned implementation**.  The directory remains loader metadata and does
+not become canonical package or import identity.
+
+#### Fresh four-axis audit and direct pre-fix measurements
+
+The bounded audit examined every permanent axis before choosing this build
+presentation difference.  Both public stages were measured at the same source
+paths:
+
+- **Go-like build:** multi-command directory selection without `-o` was already
+  aligned.  For a valid command in directory `a`, however, bare `build`,
+  `build .`, and `build ./` each exited 1 in both stages with empty stdout and
+  the same 55 stderr bytes (SHA-256
+  `2e8030a9eeb7187fbc1cb4ee9d794c352f1c31c92c884f8ae09a57e39bfe86db`):
+  `ww: build output "." already exists and is a directory`.  Explicit `-o`
+  succeeded and produced byte-identical runnable binaries, proving that only
+  default presentation was wrong.  This was the selected gap.
+- **Go-like test:** an ordinary valid zero-match directory run exited 0 in both
+  stages with identical warning, accounting, and `[no tests to run]` result
+  bytes.  The selected build branch does not enter test product naming,
+  filtering, capture, execution, result annotation, or retained test output.
+- **Go-like package:** a directory containing two selected declared package
+  names was rejected in both stages with identical diagnostics.  The selected
+  change occurs after package loading and does not alter source eligibility,
+  declaration checks, package kind, graph nodes, or actions.
+- **Go-like import:** a dotted `cyclea -> cycleb -> cyclea` graph was rejected
+  in both stages with identical cycle diagnostics.  The selected change does
+  not alter spelling, search, visibility, resolution, canonical identity, or
+  graph edges.
+
+The pre-fix statuses, streams, hashes, diagnostics, artifacts, and runtime
+results are **directly measured WW behavior**.  The cited implementation and
+testdata are **behavior directly implemented or asserted by pinned Go**.  The
+cross-axis non-effects follow from the bounded post-load output branch and are
+**behavior derived from the pinned implementation**.
+
+#### Ownership, final behavior, and preserved boundaries
+
+`cmd/ww.do_build` and `selfhost/cmd/ww.dobuild` are semantic twins and the sole
+owners of this rule.  Their current-directory predicate accepts only relative
+paths whose components are all exactly `.`.  Only in that branch do they
+canonicalize the selected directory and take its final component for output and
+cold scratch presentation.  Ordinary literal directory operands retain their
+lexical leaf, and contextual roots retain their dotted identity leaf.
+
+Direct post-fix probes through both stages show that bare, dot, and dot-slash
+builds exit 0 with empty stdout/stderr, publish mode-executable binaries named
+`a`, and produce `a.sepwork` with no `..sepwork`.  All six binaries are
+byte-identical (SHA-256
+`866c1eb875dad271d37572f43fb9d9b0eb6a2344d2e61646e655bb09f7909bf6`).
+Their unit, interface, assembly, object, archive, and init artifacts are also
+byte-identical between stages and spellings.  Current-directory library builds
+still publish nothing and link nothing; their unit, interface, assembly,
+object, and archive bytes remain stage-identical under `libcurrent.sepwork`.
+
+Loading and source selection precede this branch.  A missing dotted import
+therefore retains its byte-identical diagnostic and creates neither output nor
+scratch.  Graph and action construction, compiler/assembler/archiver/linker
+semantics, runtime behavior, and artifact content are unchanged.  A genuine
+directory occupying the derived output still rejects before tools and names
+the derived leaf in its diagnostic.
+
+Cold and warm persistent builds retain their existing keys and reuse rules.  A
+source invalidation reruns producers; an injected compiler failure preserves
+the prior executable and committed persistent generation, publishes no stage,
+and leaves no `.new`, `.install`, or `.wwtxn.*` residue.  Restoring producer
+success installs the changed executable.  Existing concurrency, interruption,
+process-group, rollback, publication, and cleanup owners gain no shared state or
+new process path.
+
+The WW-native `current_directory_build_default_output` observer proves both
+stages, all three current-directory spellings, executable mode/runtime/byte
+parity, explicit and null output controls, non-main non-publication, corrected
+cold scratch, missing-import precedence, cold/warm reuse, invalidation,
+producer-failure rollback, prior-output preservation, genuine collision, and
+residue absence.  Existing package/import graph, byte-identity, concurrent
+transaction, and interruption observers remain authoritative for mechanisms
+this presentation rule does not change.
+
+No persisted-byte contract changed.  Build workdir format remains `18`, test
+workdir format remains `19`, and semantic storage format remains `3`.
+
 ## 12. Candidate architectures and hard-gate decision
 
 Five candidates were developed as coherent systems, not as feature bins.