fix: stabilize diff panel rendering and hunk-safe truncation

[chuks-qua]

What

The diff review panel crashed with VirtualizedFileDiff.render: rendered a different diff than its prepared layout and eventually took down the renderer, and the console filled with parsePatchContent: hunk line count mismatch warnings. Two independent defects:

• ReviewDiffView re-parsed every patch into a fresh FileDiffMetadata object on any state change while stamping the same cacheKey. Pierre's areDiffTargetsEqual treats same-key objects as one render target, so the virtualizer could prepare layout for the old object and commit the new one during scroll/resize, tripping its identity assertion. Fixed with FileDiffCache, which reuses the parsed object while the patch is unchanged and gives every fresh parse a unique serial key.
• Server maxLines truncation sliced unified diffs mid-hunk, producing headers that declared more lines than were present. Pierre warned and repaired hunks with shifted boundaries. Fixed with truncateUnifiedDiff, which keeps only complete hunks at all four truncation sites (snapshot service, commit diff, working-tree diff, branch diff, including the root-commit fallback).

Why

The panel errored continuously while open until it hit the error boundary and crashed the app, and truncated inline diffs in chat rendered with corrupted hunk boundaries.

UI Changes

After: the Review panel renders the virtualized diff and survives scroll/resize/view switching with zero console errors (live-verified in Electron; screenshot could not be attached because this gh version lacks pr create --attach, and .dev/ is gitignored).

Before: renderer crashed to the app error boundary after repeated VirtualizedFileDiff.render errors (from production logs; no pre-fix screenshot captured).

Config Changes

None.

Review Notes

• Verified live in Electron: scrolled the full virtualized viewport, drag-resized the panel, resized the window, maximized/restored, switched views and files — zero diff-related console errors.
• Verified over the real WS RPC: git.workingTreeDiff with maxLines=500 returned an incomplete hunk before the fix and hunk-bounded output after.
• truncateUnifiedDiff handles \ No newline markers, @@ -1 +1 @@ countless headers, zero counts, and bare-empty context lines (diff.suppressBlankEmpty).
• Known gap (pre-existing): DiffViewer's "Show full diff" check can never trigger since the response was already capped before this change; a file whose first hunk alone exceeds the budget now renders empty instead of a corrupt partial diff. A truncation flag in the RPC contract would be the proper fix.
• Tests: packages/shared truncate-patch suite, apps/web file-diff-cache suite, plus existing snapshot/git-comparison/diff-panel suites all pass; lint and typecheck clean.

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