fix(spatch): make multi-file writes transactional

[ClaudiuCeia]

Finding

Non-interactive patching currently analyzes and writes files concurrently. A later analysis or write failure can therefore leave earlier files modified. Interactive patching preflights all selected files, but its sequential write phase can still partially apply a multi-file patch.

Portable filesystem APIs cannot provide a truly atomic project-wide commit, so the implementation must expose that limitation rather than overstate its guarantees.

Proposed approach

• Separate file analysis from filesystem mutation in packages/spatch/src/phases/rewrite.ts.
• Stage replacement and rollback content for every changed file before modifying any target.
• Preflight all targets against their analyzed source content.
• Commit in deterministic absolute-path order with a final stale-content check before each rename.
• On commit failure, roll back committed files in reverse order without overwriting concurrent external edits.
• Throw an explicit PartialCommitError listing affected files if rollback cannot fully restore the original state.
• Route interactive and non-interactive writes through the same transaction implementation in packages/spatch/src/file-write.ts.

Acceptance criteria

• Analysis, staging, or global preflight failure modifies no target files.
• Commit order is deterministic regardless of analysis concurrency.
• A later commit failure restores earlier writes when rollback succeeds.
• Incomplete rollback produces a stable typed error with deterministic affected-file paths.
• Concurrent edits are preserved rather than overwritten during commit or rollback.
• Dry-run, check mode, result shape, and CLI output remain compatible.
• Tests cover staging failure, stale preflight, commit failure with successful rollback, rollback failure, and concurrent edits.