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/ww/main.c

@@ -282,11 +282,6 @@ enumerate_dir_ww(const char *dirpath, char ***out_files)
 		size_t nl = strlen(nm);
 		if (nl <= 3) continue;
 		if (strcmp(nm + nl - 3, ".ww") != 0) continue;
-		/* skip "*.combined.ww" — legacy amalgamator artifacts
-		 * (pre-#87): they look like .ww but parse-error when
-		 * re-included, and the sep driver no longer writes them. */
-		if (nl >= 12 && strcmp(nm + nl - 12, ".combined.ww") == 0)
-			continue;
 		char path[2048];
 		snprintf(path, sizeof path, "%s/%s", dirpath, nm);
 		if ((nl >= 8 && strcmp(nm + nl - 8, "_test.ww") == 0)