test: re-point 990/994/#110 off combined.ww-input to sep-feed (M4 E4-keystone, #22)

The E3 driver flip makes build_one_sep the sole path; it writes
.sepwork/*.unit.ww, not <stem>.combined.ww, so the flip stops producing
combined.ww. Migrate its consumers to sep-feed first, while both build
paths still exist (reversible; all 5 stage binaries stay byte-identical):

- 990_selfhost: resolveunit drives `ww build --sep` and reads
  <stem>.sepwork/__root.unit.ww; probe_cgen_match gains err.ww (sep
  inlines fmt's .wwi only, so cs==ww holds); probe_ww_links re-expressed
  as `ww_ww build --sep`->run; probe_bootstrap_fixed_point removed (the
  monolith path it tested is deleted by E4 -- the self-application axis
  lives in `make bootstrap`, the cs==ww axis in 995).
- 994_w6c_ww: drop the combined-fed emit_eq + corpus; keep the live
  per-package cs-sep==ww-sep oracle (cmp_sepwork + diff_one + bad[]).
- #110 combined_ww_fresh gate removed (its regen-vs-committed premise
  dies with its producer).

Non-vacuity proven: a one-line emitter mutation reddens 994's per-package
cs!=ww check across 15 packages; revert restores green.

test/run

@@ -187,25 +187,6 @@ if [ "${UNIT:-0}" != "1" ]; then
 	done | xargs -0 -n2 -P "$JOBS" "$0" --one || true
 	p2par_end=$(date +%s.%N)
 	fi
-
-	# combined.ww freshness gate (#110): 990/995 above unconditionally
-	# regenerate every tracked combined.ww amalgamation, so a diff vs HEAD
-	# here means an embedded source changed without its derived artifact
-	# being regenerated + committed (the #28 staleness hole the per-file
-	# byte-id gates were blind to). git ls-files keeps new combined.ww
-	# auto-covered (rule-13 SSoT). Lives inside the UNIT guard because
-	# test-unit skips 990/995 and thus the regen.
-	name=combined_ww_fresh
-	if git diff --exit-code -- $(git ls-files '*.combined.ww') > /dev/null 2>&1; then
-		printf 'ok   %s\n' "$name" > "$RESULTS/$name.status"
-	else
-		{
-			printf 'FAIL %s (stale; regen + commit the derived amalgamation)\n' "$name"
-			echo '--- stale combined.ww ---'
-			git diff --name-only -- $(git ls-files '*.combined.ww')
-		} > "$RESULTS/$name.status"
-		: > "$RESULTS/$name.fail"
-	fi
 fi
 
 fail=0