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/design/multitab-pane.md

@@ -79,8 +79,8 @@ The widget is a thin shell:
 - Calls `bodyBuilder(active)` for the visible content
 - Routes user gestures to controller methods or callbacks
 - Emits `onCloseRequested` / `onAddRequested` so the host decides
-  the actual lifecycle (e.g. Claude pane spawns a new tmux session,
-  doesn't just append a UI tab)
+  the actual lifecycle (e.g. the Claude pane spawns a new stream-json
+  session, doesn't just append a UI tab)
 
 The host owns the controller and the payload type. The widget never
 touches PTY, IPC, or Claude session naming.
@@ -138,7 +138,7 @@ ClaudePane (host)
     )
 ```
 
-`ClaudeSessionRef` carries the tmux session name + isPrimary. The
+`ClaudeSessionRef` carries the stream-json session id + isPrimary. The
 controller is seeded with `[primary]` on boot; secondaries get
 appended as the user clicks `+`. Closing a secondary triggers
 `pane.close` IPC and removes the entry; closing the primary is not