test: prove case-fold collision preflight

docs/spec.md

@@ -269,8 +269,14 @@ ImportPath    = ident { "." ident } .
   are ignored. Eligible names are byte-sorted before source validation.
   Production excludes selected `*_test.ww`; test variants classify only those
   selected test files. An excluded file contributes no declarations, imports,
-  package edge, action, export, artifact, initialization, test, or persistent
-  invalidation.
+  filename collision, package edge, action, export, artifact, initialization,
+  test, or persistent invalidation. After eligibility, two distinct selected
+  basenames in one canonical directory that are equal under Go 1.26.5 Unicode
+  simple folding are rejected after the coordinator's required package-clause
+  classification and production `@test` validation parses, but before the
+  delegated graph-import scan or tools. An ordinary build compares production
+  names only; one test product compares its production, same-package test, and
+  external-test selections without merging their units.
 - `import acme.codec;` loads the canonical package `acme.codec`. If that
   package declares `package wire;`, the importing file sees its exported names
   as `wire.Name`; `codec.Name` is not an additional binding. An explicit alias
@@ -294,6 +300,12 @@ ImportPath    = ident { "." ident } .
   edge. Test-only occurrences never enter ordinary production. Self-import is
   rejected, except that toolchain-owned external-test self wiring is rebound to
   the effective augmented package action after ordinary per-site validation.
+- Canonical import identity is exact and case-sensitive. After contextual local
+  or vendor expansion, two distinct effective identities that are equal under
+  Go 1.26.5 simple folding are a request-wide structural error. Folding is only
+  a temporary collision key: it never changes lookup, action identity, `.wwi`
+  ownership, symbols, artifacts, storage, or diagnostics. Repeated occurrences
+  of the same exact identity remain valid and deduplicate normally.
 - An executable package is one declared `package main` and containing a
   `fn main`; path and directory spelling do not classify commands. An ordinary
   import of a package declared `main` is rejected, except for the toolchain's
@@ -618,6 +630,13 @@ under test binds to the augmented white-box action when it exists; affected
 transitive importers are copied and rewired so ordinary and augmented package
 state do not coexist in the linked closure.
 
+Those variants are action distinctions over exact package representatives, not
+new ordinary package identities for case-fold comparison. Production,
+same-package test, external test, and recompiled copies of one exact canonical
+package therefore do not collide with each other. Their selected source units
+remain separate, while the directory-owned filename preflight spans the
+production/internal/external selections applicable to that test product.
+
 Before a test function runs, the one generated product initializes its exact
 effective graph dependency-first and once per canonical action. Imports,
 runtime lets, and init declarations found only in `*_test.ww` never enter an