renderer: harden RenderState grapheme rebuilds

[bvolpato]

Summary

• initialize RenderState grapheme slices before taking unmarked-row fast path
• reset periodic RenderState rebuild counter after deinitialization
• add regression coverage for false-negative source row grapheme metadata

Root cause

A source row can expose a codepoint_grapheme cell while its row-level grapheme flag is false. RenderState.update copied raw cell metadata, skipped managed-memory initialization, and left Cell.grapheme pointing into a reset row arena. Link serialization later dereferenced stale slice data and crashed.

Separately, terminal_state_frame_count never reset after reaching 100,000 frames. Long-running renderers therefore deinitialized and rebuilt RenderState every frame, increasing allocator churn and exposure to stale data.

Validation

• zig fmt --check src/terminal/render.zig src/renderer/generic.zig
• ReleaseFast universal GhosttyKit build through ./scripts/reload.sh --tag fix-renderstate-grapheme-crash
• tagged cmux Debug build succeeded
• two-commit test-first history retained

Targeted regression test was not executed locally per cmux testing policy. Drafting for manual review before approval.

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Summary by cubic

Fixes a crash caused by stale grapheme slices in RenderState and stops unnecessary per-frame rebuilds in long-running sessions. We now clear grapheme slices on the unmarked-row fast path and reset the renderer’s frame counter after deinit.

• Bug Fixes
  • In RenderState.update, zero cell grapheme slices when the source row isn’t marked to avoid dangling pointers and link serialization crashes; includes a targeted regression test.
  • In the generic renderer, reset terminal_state_frame_count to 0 after deinitializing terminal_state to prevent perpetual per-frame rebuilds once the threshold is reached.

^{Written for commit 3d9239e. Summary will update on new commits.}

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