docs: dissolve PROJECT_PLAN.md; drop retired-mechanism citations

PROJECT_PLAN.md was a historical disclaimer wrapping five still-open
T1 driver items — those move to docs/test-system-v2.md (Open driver
work); the rest is dated 2026-08-05 design history, archived in git.
Also retired-mechanism cleanup: both drivers drop the *.combined.ww
enumeration skip (the amalgamator is gone; leftover debris now fails
loudly instead of being silently accommodated), the consumer-less
m4_combined_epoch.md5 pin is deleted, and the wwi.ww/ww.h/check.ww
comments that still described combined.ww as the live path are
re-cited to the sep reality.

docs/test-system-v2.md

@@ -250,6 +250,23 @@ There is no test cache, daemon, scanner, generated manifest, database, new
 framework, compatibility API, concurrency runtime, dependency, or changed
 timeout policy in this architecture.
 
+## Open driver work
+
+Carried over from the dissolved project plan; each is deliberate scope, not
+drift:
+
+- Recursive package discovery (a `./...`-equivalent pattern) for `ww test`.
+- Package-level concurrency for the coordinator's builds (it is
+  single-threaded by design today).
+- A package-level `-o` contract (single-file `ww test -o` exists; directory
+  packages publish fixed `<package>.test` stems under `-c`).
+- Universal `*_test.ww` filename migration for library tests (several
+  library suites still use the older `*test.ww` names, enumerated by
+  `LIBRARY_TESTS` and the `989_lib_byteid` roster).
+- The remaining invalid-`@test` attribute-shape diagnostics (exported
+  tests, prototypes, arguments, variadics, non-void returns, duplicate
+  annotations) with stable text in both frontends.
+
 ## Validation policy
 
 Use `test-unit` as the inner loop for lexer, parser, checker, and narrow codegen