test: prove platform filename selection

docs/test-system-v2.md

@@ -224,6 +224,19 @@ run always executes from the temp root. Without `-c`, it removes the
 temporary binary and scratch with its workspace. The language runtime
 owns individual `@test` functions.
 
+Before package grouping, both directory drivers and the shared recursive
+coordinator apply Go 1.26.5's filename OS/architecture rule for WW's fixed
+`linux/amd64` target. The basename stem ends at its first dot; a final `_test`
+token is removed for matching; a recognized OS/architecture pair takes
+precedence over a recognized final single token; and unknown or misplaced
+tokens remain ordinary. Wrong-target production, same-package test, and
+external-test files therefore create no imports, variants, generated-main
+inputs, runtime tests, artifacts, or persistent invalidation. Names are
+byte-sorted before selected-source validation, so diagnostics do not depend on
+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.
+
 Separate compilation is the only driver build path; no compatibility mode
 switch remains.
 
@@ -255,6 +268,19 @@ exact-argv, command, and persistent-workdir observers, the package suite proves
 archive-only link argv and exact warm/rejection-state behavior without
 duplicating those broader mechanisms in this observer.
 
+The same package owner contains the focused
+`platform_filename_source_selection` observer. It independently generates the
+suffix matrix, production/internal/external sources, wrong-target import and
+malformed sentinels, wrong-target-only directories, and reverse-created
+diagnostic files. Across cold and persistent Cstage/WWstage roots it compares
+normalized compiler/assembler/linker argv, units, `.wwi`, assembly, objects,
+archives, generated-main archives, binaries, runtime/test output, direct and
+recursive behavior, reversed roots, ignored-edit reuse, selected private-change
+propagation, and request rollback after a dependency has staged and the root
+compiler fails. The existing bounded-memory package-initialization observer
+continues to own allocation-failure parity for the shared dynamically grown
+action universe.
+
 `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,