Harden and land atomic downstream modification-notice tooling

[marksverdhai]

Objective

Land a reviewable tool that plans and atomically applies downstream modification notices before any repository-wide notice mutation. This supersedes the unsafe broad application in #55.

Approved decomposition

1. Read-only revision/path/header planner and policy tests.
2. Descriptor-relative mutation/rollback engine with adversarial tests.
3. Only after both land, a separately reviewed deterministic application PR.

Current local checkpoints

• planner: 4a2cab664cac337548acf4423b200962a4b40c18 (review-sized, not pushed)
• mutation WIP: local-only; still under correction

Remaining gates

• content-strong fingerprint; same-size/restored-mtime edits must not pass
• verify both displaced original and installed candidate around atomic exchange
• mismatch recovery must never delete a newer concurrent write
• rollback must verify backup content before reporting success
• --check and --dry-run must remain portable without mutation support
• no staging-file leaks on write/chmod/fsync/stat failure
• generated Python headers with encoding cookies remain excluded
• legal existing notices after copyright headers remain idempotent
• all diagnostic line separators escaped and aggregate output bounded
• focused adversarial tests, Ruff, formatter, and diff checks

No mass source-file mutation is authorized by this issue.