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.

test/wcc/m4_combined_epoch.md5

@@ -1,14 +0,0 @@
-# m4-combined-epoch — combined≡sep boundary, proven by the c6 gate (994 Tier-C)
-# emitter: out/bin/w6c_ww md5=840025113b9d72bee4402a011369336f (combined-built)
-# regen: git checkout m4-combined-epoch && make && for t in w6c wwdump w6a w6l ww; do out/bin/w6c_ww -o /tmp/e0_$t.s selfhost/cmd/$t/main.combined.ww; md5sum /tmp/e0_$t.s; done; diff vs below
-bf3c25ed79da65520e049c6d657fc482  w6c.s
-f8d37b1637fd03a93f1a4b2ea801037b  wwdump.s
-aa9a76494f8b0cef9565bba857c7ff76  w6a.s
-97b987dc914f93846ffbfe446cd47c92  w6l.s
-235ff74409ad1c2359135f455af507fe  ww.s
-# combined-epoch binary pins:
-840025113b9d72bee4402a011369336f  w6c_ww
-b8c8c57dd25dbb5d03dc424740f52c83  wwdump_ww
-2677b37ad0a0d47517e7efd5f1824aba  w6a_ww
-ec686a46a586931e3607617395116d27  w6l_ww
-8cb2aec5cf76fbc7d6c1e37808add6ec  ww_ww