perf: structural sharing in setAppState + React.memo on the pane tree

[Zeus-Deus]

Fixes #127.

Problem

The whole active pane tree re-rendered on every backend tick. The Rust backend rebuilds the entire AppStateSnapshot on every emit_app_state (coalesced to 16ms windows, so up to ~60 full snapshots/sec under agent streaming), the frontend debounces another 16ms, and then setAppState swapped the snapshot in with no structural sharing — fresh refs for every workspace/surface/pane meant useActiveWorkspace() churned every tick and the un-memoized WorkspaceMain → PaneContainer → PaneNode → TerminalPane chain reconciled top to bottom.

Part 1 — structural sharing in setAppState

New src/lib/structural-share.ts: shareStructural(prev, next) reconciles the incoming snapshot against the one already held, reusing the previous refs for every deep-equal subtree (arrays element-wise, plain objects per key, recursive through the pane tree). A no-op tick returns the previous top-level ref → zero zustand fan-out; a change to one workspace's pane_statuses leaves every other workspace object and every unchanged surface/pane subtree identity intact. useActiveWorkspace()/useActiveSurface() become stable across no-op ticks with no call-site changes, and the WeakMap-cached buildSessionWorkspaceIndex stays correct (reused ref ⇒ unchanged content).

Deliberately unlike the removed full-snapshot JSON.stringify dedup (reverted for causing freezes — see the comment in use-app-state.ts): the walk allocates no strings, runs in a single O(n) pass with per-branch early exits, and executes at most once per 16ms debounce window. Worst case it's one walk of a snapshot that was just JSON-deserialized over IPC anyway.

Part 2 — memoize the pane tree

React.memo on PaneContainer, PaneNode (the split branch's recursion resolves to the memoized component), TerminalPane + BrowserPane (export-only wrappers — the xterm lifecycle, write pump, and PTY path are untouched, per the perf-regression history), and WorkspaceMain's heavy children TabBar, PresetBar, RightPanel. Part 1 is what makes these effective.

Also fixes a latent rules-of-hooks bug in PaneNode: useAppStore/useFeatureFlags were called after the kind === "split" early return, so a split↔leaf flip at the same fiber position would change hook order and throw. Both hooks are hoisted above the branch.

Verification

• npm run verify green: cargo check + cargo test, tsc, 165 test files / 2383 frontend tests.
• New src/lib/structural-share.test.ts (18 cases): identity fast paths, deep-equal → prev ref, array grow/shrink/reorder, Record key add/remove/swap, optional-field presence, recursive split-tree branch sharing, input immutability.
• New store-level tests: setAppState(deepClone(A)) keeps the previous snapshot ref; a one-workspace change keeps every other workspace's identity.
• New src/components/layout/pane-tree-rerender.test.tsx (5 cases) drives the real store → useActiveWorkspace() → memo chain under a <Profiler>: zero commits on a deep-equal tick and on an inactive-workspace change; a real pane-title change and a split→single structure change still render correctly (guards against over-memoization).
• Live pass in the dev-mock UI (npm run dev + browser pane): workspace switch, workspace close, and chat/terminal surfaces all update immediately. Pane split/close/tab mutations have no dev-mock handlers (real-IPC only) — those paths are covered by the integration suite, and their backend emits are direct (emit_app_state), not coalesced, so no added latency.
• No Rust changes; nothing on the keystroke echo path.