feat(tests): caption posted screenshots (name + spec); raise drift floor to 0.5%

Bake a caption strip onto every posted screenshot composite — status + name +
the spec that produces it — for changed, added, and removed; removed now shows
the old baseline image (was a text-only line). Zero native-dep: an embedded
public-domain 8x8 bitmap font (font8x8.ts) rendered by image-ops `withBottomLabel`;
the name→spec attribution is factored out of gen-manifest into a shared
`spec-map.ts` resolver. Bottom strip, colour per status (green/red/orange).
Applies in both compare (drift gate) and changelog (git-history diff), and
post-discord now attaches the captioned removed images.

Also raise the per-engine drift floor 0.2% → 0.5% (changedRatio) to absorb the
sub-1% inter-run flakiness seen after the re-baseline, while still catching real
localized changes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/screenshot-manifest.json

@@ -1,5 +1,5 @@
 {
-  "_note": "engine tags are best-effort (gen-manifest.ts); the name list is authoritative. Refine engines after calibration.",
+  "_note": "engine tags are best-effort (spec-map.ts); the name list is authoritative. Refine engines after calibration.",
   "screenshots": [
     {
       "name": "01-loading.png",