feat(projects): render diffs with Pierre

[Su3h7aM]

Summary

Replace the Projects hand-written diff renderer with the Pierre diff rendering stack while preserving Buzz review behavior, then stabilize PR and commit diff workspace geometry across wide, stacked, and narrow layouts.

The four reviewed commits are based directly on current upstream main (c923e89a4b6d43ae0c507dbb5e58f2bdd9ab7888).

Changes

• Add @pierre/diffs and pure Buzz↔Pierre annotation mapping helpers for path, side, line, focused range, renderability, and patch-body accounting.
• Add the shared ProjectDiffBody adapter with Buzz theme integration, exact inline-comment anchors, focused selection, normalized unified patches, truncation handling, and readable fallback behavior.
• Adopt the adapter in the shared Projects changed-files panel, preserving file selection/filtering, headers and totals, one active review form, normal-pointer inline commenting, focus clearing, request-changes history, commit-detail reuse, and accessibility.
• Stabilize the Projects layout chain: normal wide bottom gutter, independently scrollable file tree and diff body, reachable final tree row, aligned PR main/meta rails, stacked rail flow, and content-driven narrow layouts.
• Add deterministic Projects fixtures and E2E contracts for TSX, YAML, unknown-extension plain text, buzz and buzz-dark themes, PR geometry, commit geometry, exact selected paths/content, and fallback behavior.

Validation

• Focused annotation/focus tests: 32/32 passed
• Full desktop unit suite: 4567/4567 passed on the reviewed stack before the final geometry assertion/correction batch
• Complete Projects E2E (project-pr-review + project-commit-detail): 39/39 passed on final content
• pnpm typecheck: clean on final content
• Biome on all touched final-correction files: clean
• check:px-text and check:pubkey-truncation: passed in the full stack gate
• Production build: clean in the full stack gate; final bounded geometry correction was subsequently covered by typecheck and the complete Projects E2E run
• Direct governed file-size checks: within the repository limits for governed source files

Review state

• T1, T2, F1, and F2 were independently reviewed and carry the actual reviewer credits.
• The four-commit stack is conflict-free and contains exactly 15 intended files relative to upstream main.
• This PR is intentionally a draft while follow-up work continues.