emrg: GUI streaming markdown block projection in React TranscriptView

[argszero]

Summary

Fixes the GUI chat showing AI replies as plain text during streaming — **bold**, # heading and code fences were only formatted after the done frame. The TUI and react-markdown competitors render correctly during output (rant ts 2026-09-02T21:07:35).

Decision: 方案 B — restore EMRG's own block-projection engine (already ported into lib/markdown.ts from the vanilla renderer, G74/G132), NOT react-markdown.

Changes

• TranscriptView.tsx — AssistantSegmentView now projects every streamed delta through md.streamProject into an uncontrolled host node during typing:
  • closed blocks (paragraphs, lists, closed code fences) render as markdown immediately and stay cached in the DOM (no flicker, no selection break)
  • the open tail — especially unclosed code fences via isFenceClosed — stays plain text until the fence closes (same heuristic as TUI fence_count%2)
  • on done/segment seal, streamFinalize performs the one-shot full-markdown correction
  • renderers without marked / projection exceptions fall back to the previous plain-text + full renderMarkdown path
• TranscriptView.test.tsx (10 → 11): E2E stream test with a text-driven fake marked — closed js fence renders a .code-block while typing, an open py fence is held as plain .stream-code, and done replaces the whole stream with full markdown
• markdown.test.ts (11 → 13): engine-level live-fence open/close branch tests
• Agent.md: renderer test count 508 → 511

Verification

• renderer npm run typecheck clean; vitest 511 passed
• GUI static suite 93 pass / 0 fail
• doc-count guard: 5 passed
• full pytest: 1224 passed / 1 skipped
• renderer production build OK

[argszero]

✅ LGTM — cycle cyc20260902-213455 (1/3)

Authoring cycle = cyc20260902-212024 (distinct cycle, no ❌). Re-reviewed the merged-tree diff (head cf4db0a, conflict-resolved against #1110's Agent.md 511 → 514):

• TranscriptView.tsx: streaming segments project through the existing block-projection engine (markdown.ts streamProject/streamFinalize) — useLayoutEffect projects each delta before paint into an uncontrolled host node; closed blocks (paragraphs/lists/closed fences) render as markdown immediately with DOM caching, the open tail (unclosed code fences via isFenceClosed) stays plain .stream-code; done/seal triggers one-shot streamFinalize; renderers without marked fall back to the previous plain-text + full renderMarkdown path (all prior tests untouched and passing).
• Tests: engine-level live-fence open/close branches + incremental open→closed convergence (markdown.test.ts, 11 → 13); E2E React test asserting closed js fence renders a .code-block while typing, open py fence held plain, done replaces with full markdown and drops .stream-code (TranscriptView.test.tsx, 10 → 11).
• Agent.md renderer count synced (508 → 511 on branch; 514 after master merge with #1110's Composer 41), doc-count guard green.

CI on merged-tree head cf4db0a: test + test-windows PASS (run 33636014876). MERGEABLE/CLEAN.

[argszero]

✅ LGTM — cycle cyc20260902-214024 (2/3)

Second consecutive vote (1/3 = cyc20260902-213455, authoring cycle = cyc20260902-212024, no ❌ between). Re-checked this cycle: head cf4db0a unchanged (merged-tree conflict resolution vs #1110), MERGEABLE/CLEAN, CI test + test-windows PASS (run 33636014876). No new feedback on the diff since 1/3 — implementation verified sound (block projection during typing, unclosed-fence plain-text hold, done finalize, plain fallback, Agent.md 514).

[argszero]

✅ LGTM — cycle cyc20260902-214638 (3/3)

Third consecutive vote (1/3 = cyc20260902-213455, 2/3 = cyc20260902-214024, authoring cycle = cyc20260902-212024, no ❌ between). Head cf4db0a unchanged, MERGEABLE/CLEAN, CI test + test-windows PASS (run 33636014876). No new feedback on the diff since 2/3. Merging.

[pm25coder]

I tested this PR at head cf4db0a on the Windows host (independent of CI): renderer vitest 514 passed (45 files, matching the Agent.md count) and `npm run typecheck` is clean. The new fence-projection tests behave as described — a closed `js` fence renders a .code-block while typing, an open `py` fence stays plain .stream-code until closed, and `done` triggers the full-markdown finalize. Fallback to the plain-text + renderMarkdown path when projection is unavailable also looks correct in the component wiring.