perf(desktop): incremental markdown rendering during streams

Re-parsing the full message markdown every reveal frame is O(N^2) over a
long answer and dominated stream CPU.

- Throttle useSmoothReveal commits to ~1 frame (REVEAL_MIN_COMMIT_MS).
- Memoize block parsing with an LRU keyed on source text so only changed
  blocks re-parse.
- Replace Streamdown's full-text parseIncompleteMarkdown with a
  tail-bounded remend: scan to the last top-level boundary outside
  fences/math and repair only the trailing open block. New remend-tail.ts
  is proven render-equivalent to full remend at every streaming prefix
  (remend-tail.test.ts), minus an intentional, documented divergence on
  cross-block dangling openers.

package-lock.json

@@ -119,6 +119,7 @@
         "react-router-dom": "^7.17.0",
         "react-shiki": "^0.9.3",
         "remark-math": "^6.0.0",
+        "remend": "^1.3.0",
         "shiki": "^4.0.2",
         "streamdown": "^2.5.0",
         "tailwind-merge": "^3.5.0",