test: prove case-fold collision preflight

docs/test-system-v2.md

@@ -242,6 +242,22 @@ directory entry order. A recursive pattern omits a directory with no eligible
 source, while an explicit wrong-target-only build rejects it as having no WW
 package source. There is no source-level build-expression or user-tag mode.
 
+After that eligibility boundary and the coordinator's required package-clause
+classification and production `@test` validation parses, the delegated loader
+performs selected-basename Go 1.26.5 simple-fold preflight before its graph
+import scan. During graph loading, effective canonical imports register only
+after contextual local/vendor expansion; exact identity reuse is accepted, but
+two distinct fold-equivalent identities reject the entire shared plan.
+Selected basenames register in one canonical-directory scope. A production
+build registers production names only. A test plan shares the filename scope
+across production, internal-test, and external-test actions, including
+same-only, external-only, and mixed directories, without combining their source
+units. Hidden, underscore-prefixed, wrong-platform, and otherwise ineligible
+files never register. Package variants, product-scoped recompiled copies,
+support, and generated main remain action wiring over exact identities and do
+not manufacture package collisions. The fold preflight does not replace or
+precede those coordinator-owned source-validation parses.
+
 Separate compilation is the only driver build path; no compatibility mode
 switch remains.
 
@@ -294,6 +310,24 @@ have the same `ww: out of memory` diagnostic, invoke no compiler, assembler, or
 linker, and leave an empty caller work directory with no output, status, or
 staging path.
 
+The same observer now owns selected-basename folding: ASCII production,
+internal, external, same-only, external-only, mixed, and cross-classification
+collisions; printable Unicode Kelvin folding; malformed UTF-8 replacement-rune
+keys with exact `\xNN` diagnostics; accepted composed/decomposed neighbors;
+ordinary-build isolation from test collisions; ignored folded pairs; reversed
+creation order; and direct/recursive collision-diagnostic parity. It proves zero
+compiler/assembler/linker calls and an empty workdir, including no
+test-support/main artifact, on cold rejection. A committed warm package then
+gains and loses a colliding file; the failed middle request preserves every
+unit, `.wwi`, assembly, object, archive, binary, tool record, and stamp, and
+removal restores exact reuse.
+`package_graph_diagnostics_are_stable` provides the corresponding direct,
+transitive, reversed-root, recursive, same-directory, exact-repeat, and warm
+import proof. `vendor_directory_import_resolution` proves folding uses the
+expanded vendor identity under reversed imports and product descriptors, even
+when both spellings reach one physical directory, and that multi-product
+rejection publishes no output, status, tool state, or staging residue.
+
 `test/sep/sepinit_test.ww` is the single focused package-initialization owner.
 It generates all source trees temporarily and runs independent cold/persistent
 Cstage and WWstage legs. Its matrix covers blank-only reachability; dependency,
@@ -353,6 +387,13 @@ real directory test products always run, while no-selected-test directories do
 not create a process. The byte-identity and bootstrap gates keep building on
 fresh scratch. `make clean` reclaims every workdir under `out/`.
 
+Fold keys and first-spelling tables are request-only loader state and are never
+stored in a workdir. They do not alter successful unit bytes, action/storage
+keys, or tool records, so build format 18, test format 19, and semantic storage
+format 3 remain unchanged. Structural fold rejection occurs before staging and
+cannot mutate a committed generation; a later request with the collision
+removed evaluates the original exact-key reuse contract.
+
 On a package or tree target, `ww build/test -w DIR` forwards that exact caller
 path as the one command-global semantic-action store; neither pattern spelling
 nor package-group name derives a persistent subdirectory. The delegated driver