docs: correct stale architecture references

Found by a docs/ staleness audit:
- architecture.md: Claude no longer runs under tmux — it's driven over
  the stream-json control protocol with --resume (D-75/D-77/D-78); and
  the IPC socket server is implemented, not "currently unimplemented".
- testing/README.md + claude-ui-workflow.md: drop the dissolved app/
  two-package paths (D-56) — tests live at test/ and the web build at
  build/web/.
- design/multitab-pane.md: the Claude pane spawns a stream-json session,
  not a tmux one; ClaudeSessionRef carries the session id.

Frozen historical snapshots (initial-plan.md, the HISTORICAL pty docs,
dated spikes/audits) left as-is.

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

docs/testing/claude-ui-workflow.md

@@ -4,7 +4,7 @@ Claude Code needs to *actually use* the app while building features.
 The pipeline:
 
 1. **Flutter builds the app to WASM.** `flutter build web --wasm` ships
-   a CanvasKit/Skwasm bundle under `app/build/web/`.
+   a CanvasKit/Skwasm bundle under `build/web/`.
 2. **A local server serves it.** `tools/ui/serve.sh` starts
    `http://localhost:4280` in the background with a pidfile.
 3. **Playwright drives a headless Chromium.** Instead of click-by-pixel