lib/regex: rehome private-touching @tests to in-package regex_whitebox.ww

White-box @tests move into a colocated 'package regex' file (Hare
+test.ha analogue; #6 non-T drop plays the build-tag role), black-box
@tests stay in package regex_test — Go's foo/foo_test split. The
wb/ stub driver is documented-temporary scaffolding: a same-dir
importer file-resolves to regex.ww before dir-enumeration, so only a
different-dir import bundles the whitebox sibling (task #33 retires
it). CLAUDE.md rule-9 carve-out updated (#5 closed, gate was #6).

CLAUDE.md

@@ -6,7 +6,7 @@
 6. ref/hare and ref/plan9front are read-only references — consult before inventing data shapes or syntax
 7. No workarounds. If a bug forces a workaround, STOP and report with a precise repro. Document any retained divergence at the site with a pointer to the filed task. Never silent.
 8. Comments are WHY-only. Never narrate WHAT the code does — names carry the WHAT. Comment only non-obvious WHY: a constraint, a divergence from a reference, a citation to a filed task.
-9. Hare-fidelity over convenience. No ad-hoc extensions, renames, or convenience wrappers in lib/. Cite ref/hare/<module>/<file> for every signature ported. Carve-out: the Hare `_unsafe` suffix convention is dropped wholesale (ww is C/Plan-9-lineage, an unmanaged systems language — no GC, no "safe" baseline to be unsafe relative to). bytes→str is a pure reinterpret (`strings.frombytes`, renamed from Hare's `fromutf8_unsafe`); validation is opt-in via `utf8.validate(b)?` at the IO source, never wrapped per-construction. Rationale: ref/hare/strings/utf8.ha:10,22 — the suffix flags Hare's managed-bytes-safety axis, which ww doesn't have. The honest name (`frombytes`) reserves `fromutf8` for a future true validating helper. Carve-out: lib/ tests use Go's external-test-package idiom (`package <mod>_test;` + `import <mod>;`), a sanctioned Go-over-Hare departure — ww hard-rejects self-import (full Go), and Hare's same-package `@test` colocation has no ww analogue yet (in-package white-box testing is deferred pending tasks #5/#8, not forbidden). Rule 5/9 binds lib/ *API* surface, not test-authoring style. Ref: Go `foo_test` convention; task #16.
+9. Hare-fidelity over convenience. No ad-hoc extensions, renames, or convenience wrappers in lib/. Cite ref/hare/<module>/<file> for every signature ported. Carve-out: the Hare `_unsafe` suffix convention is dropped wholesale (ww is C/Plan-9-lineage, an unmanaged systems language — no GC, no "safe" baseline to be unsafe relative to). bytes→str is a pure reinterpret (`strings.frombytes`, renamed from Hare's `fromutf8_unsafe`); validation is opt-in via `utf8.validate(b)?` at the IO source, never wrapped per-construction. Rationale: ref/hare/strings/utf8.ha:10,22 — the suffix flags Hare's managed-bytes-safety axis, which ww doesn't have. The honest name (`frombytes`) reserves `fromutf8` for a future true validating helper. Carve-out: lib/ tests use Go's external-test-package idiom (`package <mod>_test;` + `import <mod>;`), a sanctioned Go-over-Hare departure — ww hard-rejects self-import (full Go), and Hare's same-package `@test` colocation now has a ww analogue: in-package white-box `@test` files (`package <mod>;`, colocated) are sanctioned alongside the external black-box `package <mod>_test;` split — Go's `foo`/`foo_test` model — now that the non-T `@test` drop landed (#6, harec check.c:3941, 08a76cf; #5 closed). Rule 5/9 binds lib/ *API* surface, not test-authoring style. Ref: Go `foo_test` convention; task #16.
 10. Symmetric stages. cstage and wwstage MUST emit byte-identical asm for the same input. When inference power differs, align the richer side DOWN to the leaner side, not the other way.
 11. Split commits when they bundle unrelated concerns. Bisect-cleanliness is the default. Multi-fix commits need a body paragraph explaining why they couldn't split.
 12. Simple data, simple algorithms. Sea-of-stars style. Mirror Hare's structural choices over clever alternatives.