Merge origin/main (v1.80.0.0) into consolidate-browser-skills-into-aside; queue-advance release to v1.81.0.0

Both sides claimed v1.80.0.0, so VERSION, package.json and the agents digest
auto-merged without a conflict; the release is re-versioned to the next free
MINOR slot (bin/gstack-next-version), the size-budget baseline renamed to
match, and the CHANGELOG carries the Aside-first entry above main's.

Two semantic conflicts hidden in the clean setup auto-merge are resolved here:
- _prune_stale_generated deleted a REAL host directory on banner-only proof;
  main's #2119 gate makes that weak proof file-scoped, so real dirs now go
  through _cleanup_weak_dir (SKILL.md, marker and our links only).
- _browser_hint and the Chromium bootstrap summary now consult _PW_FAIL_REASON
  and Aside presence, so they never promise a bundled browser that cannot
  launch and never tell an Aside user their browser skills are gone.
README, TESTING_INTERNALS and TODOS wording reconciled with the merged tree.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

test/helpers/carve-guards.ts

@@ -478,7 +478,7 @@ export const CARVE_GUARDS: Record<string, CarveGuard> = {
     },
     behavioral: 'prompt',
     maxSkeletonBytes: 74_500, // + Aside browser contract for Step 7 canary ({{ASIDE_SETUP}}); measured 73_523
-    maxSizeRatio: 1.10, // + v1.80 Aside contract + gstack-browser fallback block; measured 1.077
+    maxSizeRatio: 1.10, // + v1.81 Aside contract + gstack-browser fallback block; measured 1.077
     minUnionBytes: 91_000, // Phase 4 wave 1; estimated union ~94.9KB
     mustContain: ['readiness', 'merge', 'canary', 'revert', 'staging'],
   },
@@ -607,7 +607,7 @@ export const CARVE_GUARDS: Record<string, CarveGuard> = {
     },
     behavioral: 'prompt',
     maxSkeletonBytes: 63_500, // + v2.0 {{ASIDE_SETUP}}/{{BROWSE_FALLBACK}} (replaces the browse setup block); measured 61_253
-    maxSizeRatio: 1.08, // + v1.80 Aside contract + gstack-browser fallback block; measured 1.063
+    maxSizeRatio: 1.08, // + v1.81 Aside contract + gstack-browser fallback block; measured 1.063
     minUnionBytes: 69_500, // measured union 70,385
     // 'aside repl' pins the Aside contract; '$B goto' pins the fallback block in the always-loaded skeleton.
     mustContain: ['bug', 'aside repl', '$B goto', 'fix', 'Health Score Rubric', 'regression'],