fix(spatch): make multi-file writes transactional

[Copilot]

Non-interactive writes analyzed and wrote files concurrently, so a later write failure left earlier files modified. Interactive mode preflighted selected files but its sequential write loop could still partially apply across files.

Changes

file-write.ts — transaction primitives

• Export FileWriteFs (was unexported)
• Add TransactionEntry — staged write descriptor (filePath, originalText, rewrittenText, encoding, operationName)
• Add PartialCommitError — typed error with deterministically sorted affectedFiles: readonly string[]; thrown when rollback cannot restore all committed files
• Add commitTransaction(entries, options?):
  1. Sort entries by absolute path for deterministic commit order
  2. Preflight all targets (read + compare to originalText) — any mismatch aborts before any write
  3. Commit sequentially using the existing atomic write-rename
  4. On failure: rollback committed files in reverse order, skipping files with concurrent external edits (preserves them)
  5. Throw PartialCommitError on incomplete rollback; otherwise rethrow the original commit error

rewrite.ts — separate analysis from mutation

• Split rewriteFile into analyzeFile, which reads/matches/renders but does not write
• analyzeFile returns { fileResult, writeEntry } — writeEntry is null for dry-run or no-change files
• rewriteProject runs analyzeFile concurrently, collects TransactionEntry items, then calls commitTransaction once — any analysis failure aborts before any file is touched

run.ts (interactive) — route through shared transaction

• Replace the per-file writeFileIfUnchangedAtomically loop with a single commitTransaction call over all prepared files

Tests

• 8 new unit tests for commitTransaction: empty entries, multi-file write, deterministic order, stale preflight abort, commit failure + successful rollback, rollback failure → PartialCommitError, concurrent edit preserved, error shape
• 2 new integration tests for multi-file non-interactive and interactive transactional write

Analysis failure  → commitTransaction never called → no files touched
Stale preflight   → aborts before first write      → no files touched
Commit failure    → rollback in reverse order      → committed files restored
Rollback failure  → PartialCommitError(affectedFiles: sorted absolute paths)
Concurrent edit   → rollback skips that file       → external edit preserved

• Fixes fix(spatch): make multi-file writes transactional #8