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.

selfhost/cmd/wcc/check.ww

@@ -3780,7 +3780,7 @@ fn exprtype(c: *checker, e: *syntax.node, hint: *syntax.node) *syntax.node = {
 				// `scopelookup(c.cur, nm)` else-arm bound whichever
 				// same-leaf global fn headed the flat scope bucket —
 				// order-sensitive (os.read:i64 vs io.read:(i32|eof|closed)
-				// flipped by combined.ww concat order), yielding a
+				// flipped by scope-install order), yielding a
 				// false `return: not assignable`. Leaving s nil falls
 				// through to the fn-VALUE path below (exprtype(callee) →
 				// TPTR peel → TFN.ret), matching cstage cmd/wcc/check.c