perf(chat): bound Pierre file-pair rendering

[Pearcekieser]

Problem / Motivation

Large raw old/new file pairs can block the dashboard renderer before Pierre's worker pool or row virtualizer can help. A captured client trace showed three Pierre/Shiki main-thread tasks lasting 4.40 seconds, 2.49 seconds, and 2.41 seconds while entering Chat, switching sessions, and rendering paginated history.

Pierre 1.3.5 constructs file-pair metadata synchronously through MultiFileDiff -> parseDiffFromFile -> createTwoFilesPatch -> processFile. Worker-backed highlighting does not bound that initial work.

Why it matters

The measured tasks accounted for 90.2% of long-task time, produced a 4.55-second visible freeze, and delayed one interaction by 2,545 ms. Code-heavy chats become unresponsive during navigation even when the destination response has not arrived yet.

What changed (motivation → approach → change)

The public PierreFilePair wrapper now owns a layout-independent input budget before its lazy Pierre chunk loads.

• Pairs within 400 lines per side and 131,072 combined UTF-16 code units keep the existing Pierre renderer.
• Larger pairs bypass Pierre and show both complete files in a themed plain view.
• The fallback preserves header slots, collapsed rows, selection, wrapping, split and unified layouts, narrow stacking, and the existing 376 px chat-row scroller.
• Scrollable source regions are keyboard reachable.
• A translated Large file — simplified view status explains why highlighting and line-level diff controls are absent.
• Oversized live-edit buffers remain editable through the normal file editor without live diff decoration.
• Source-only rendering and already-computed unified patches are unchanged.

Relationship to #7916

PR #7916 overlaps FileChangeChips, PlainCodeFallback, and the Pierre wrapper as part of a broader history-pagination, staged-rendering, and phone-scroll rewrite.

This PR is intentionally based directly on main, not on #7916. The two changes address different costs. #7916 changes when transcript and diff work becomes reachable; this PR places a measured hard bound on synchronous raw file-pair construction whenever a pair is rendered. Expanded rows, side panels, and live-diff editing still need that bound after staged mounting or history pagination lands.

Shipping the guard directly keeps a separate regression and rollback boundary for the measured freeze. Waiting for or stacking on #7916 would couple a focused renderer safety limit to a much larger in-flight history rewrite and duplicate its review dependencies. Any textual conflict can be resolved when main moves without changing this PR's policy or tests.

Tests

• pierre.renderBudget.test.ts pins exact line and UTF-16 code-unit boundaries, uneven pairs, and new-file behavior.
• PierreFilePair.budget.test.tsx proves oversized pairs never mount Pierre while preserving both files, header defaults, collapsed controls, keyboard regions, separators, and caller sizing.
• PierreEditorImpl.test.tsx proves oversized live diffs remain editable.
• FileChangeChips.headerSlots.test.tsx pins the shared 376 px fallback scroller.
• Fresh-worktree focused run: 60 tests passed across 4 files.
• TypeScript, ESLint, full i18n checks, and the production build passed.
• Two independent pre-publication reviews found six fallback parity and accessibility issues; all were fixed and reverified.

Manual verification

A real Chromium fixture rendered a synthetic 401-line pair and verified:

1. The simplified view appears instead of Pierre rows.
2. Both complete sides remain selectable.
3. The content stays inside the 376 px internal scroller.
4. Two keyboard-focusable source regions are present.
5. Desktop uses side-by-side panes and narrow width stacks them.
6. No page errors occur.

Screenshots / video

Desktop split view

Narrow stacked view

Related Issues

Fixes #8241

Possible overlap: #7916, as described above.

Pattern harvest

Rule candidate: review-prompt
Pattern: Worker offload does not bound synchronous preprocessing that runs before the worker request; expensive renderers need an app-owned input budget at their public boundary.

Checklist

• At most two commits (one is the norm), with a Conventional Commits title (feat|fix|docs|refactor|perf|test|chore|ci|build|revert: ...)
• Existing tests pass and new tests added for new functionality
• Self-review completed; code follows project style guidelines
• Documentation updated (if applicable)
• No secrets, credentials, or internal references in the diff