Cover empty-message broadcast fallback and persist mutation state

Add a unit test for the broadcast-error message fallback to kill the last
new-post mutant, and commit the differential and acceptance mutation
manifests written by the approved tools during the architect verification
pass for the new-post-errors feature.

By architect.

docs/reviews/new-post-errors-summary.md

@@ -0,0 +1,60 @@
+# Architectural Review Summary — new-post-errors
+
+## Task and commits reviewed
+- Task: `new-post-errors`
+- Reviewed the merged branch ending at `66a684923c` (refactorer), which carried:
+  - `a125751`/`4e0f40c` — specifier broadcast-error surfacing spec (`specs/memo-new.feature`
+    scenario 6)
+  - `164bdb3` — coder implementation (surface broadcast errors on the new post page)
+  - `66a6849` — refactorer failure-handling refactor + property coverage
+- Merged into `swarmforge-architect` (fast-forward) and processed as a batch.
+
+## Architectural findings and fixes applied
+Reviewed UI/Core separation, dependency rule, information hiding/encapsulation, and
+local code quality.
+
+1. **Failure classification (good).** `src/services/new-post.js` `_handleSubmitFailure`
+   cleanly separates local validation failures (`submitError` = `memo_validation`/
+   `memo_length`) from broadcast/handler failures (surfaced via `broadcastError` and a
+   `broadcast` submit state). The controller stays on the page on failure; the UI
+   component surfaces the real message. Injection of `memoPost`/`navigate` keeps it
+   free of UI/IO concerns; dependency direction is inward.
+2. **Unified handlers extended (good).** `acceptance/lib/handlers.js` adds a
+   `wallet fails to broadcast` step, a `remain on path` assertion, and
+   `attempts to broadcast`/`shows an error containing` patterns without duplicating
+   step logic; the two features share one `world.newPage`.
+3. **Property coverage (good).** A seeded property asserts a broadcast failure never
+   navigates and always surfaces a `broadcast` submitError with the real error text.
+4. **Fix applied — error-message fallback coverage.** The language mutation tool
+   surfaced one survivor on `new-post.js:87` (`err.message || String(err)` → `&&`),
+   which only matters when `err.message` is falsy. No test exercised that path. Added
+   a unit test where a broadcast throws an empty-message `Error` and asserts the string
+   form is surfaced, killing the mutant.
+
+## Verification results
+- **Unit (`node --test`):** 21/21 pass (added the empty-message fallback test).
+- **Property (`npm run test:property`):** 7/7 pass.
+- **Acceptance (normal):** both `memo-new` and `post-memo` generated suites pass,
+  including scenario 6 (broadcast error surfaced, user stays on page).
+- **Mutation (`mutate4javascript`, `--max-workers 8`, `--mutate-all`):**
+  `new-post.js` — **killed 12 / survived 0 / uncovered 0** (was 1 survivor before the
+  fix).
+- **DRY (`dry4javascript src`):** no duplicate candidates.
+- **Gherkin acceptance mutation (soft):** `memo-new.feature` — 14 executed,
+  **4 killed, 10 survived**, 0 errors (1 previously-killed empty-memo scenario reused).
+  - Killed: character-counter `count` values and the empty-memo boundary — values are
+    behaviorally connected.
+  - Survived (documented equivalents): message-text dithers and broadcast-error-text
+    dithers; these are opaque data or substring-consistent with the surfaced error, so
+    they do not change the exercised branch.
+- Property tests run separately via `npm run test:property`.
+
+## Suite status
+- Unit + property + acceptance all pass; source-level mutation fully kills both testable
+  core modules. Gherkin acceptance mutation survivors are documented equivalents.
+
+## Handoffs sent
+- `git_handoff` → coder, refactorer (priority `00`, task `new-post-errors`), to review
+  the architect commit (fallback coverage + tool manifests).
+
+By architect.