test: prove platform filename selection

docs/spec.md

@@ -255,6 +255,20 @@ ImportPath    = ident { "." ident } .
   `.ww` files sharing one declared package name compiles as one package. The
   declared name need not equal the directory name or the final component of its
   canonical import identity.
+- Directory source eligibility uses Go 1.26.5 filename suffix semantics for
+  WW's fixed `linux/amd64` target. In the basename stem before the first dot, a
+  final `_test` token is ignored for platform matching. A final known OS or
+  architecture token must match `linux` or `amd64`; a final known OS followed
+  by a known architecture takes precedence and both must match. The known-name
+  sets are Go 1.26.5's `syslist.KnownOS` and `syslist.KnownArch`. Unknown or
+  misplaced suffixes are ordinary, and a platform word without a nonempty
+  underscore prefix is ordinary (`linux.ww` and `plan9_test.ww` are selected;
+  `x_windows.ww` and `x_plan9_test.ww` are not). Leading-dot/underscore entries
+  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.
 - `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