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.

cmd/wcc/ww.h

@@ -350,11 +350,13 @@ struct Node {
 	Type		*type;	/* filled in by checker */
 	const char	*tsuffix;	/* typed numeric literal suffix */
 	const char	*module;	/* `// MODULE: foo` directive at the
-					 * decl's section in combined.ww.
-					 * NULL for nested nodes; only top-
-					 * level decls (fn/def/type/let)
-					 * carry it. On an N_USE node this is
-					 * the importing (owning) module. */
+					 * decl's section of a resolved
+					 * compilation unit (the sep driver's
+					 * .unit.ww). NULL for nested nodes;
+					 * only top-level decls (fn/def/type/
+					 * let) carry it. On an N_USE node
+					 * this is the importing (owning)
+					 * module. */
 	const char	*usepath;	/* M1 #22: on an N_USE node, the full
 					 * dotted IMPORT path (`encoding.utf8`)
 					 * vs the leaf alias in `str`. Drives