After every board-mutating SWIG command, kicad_interface._auto_save_board()
unconditionally calls pcbnew.SaveBoard() with the in-memory board. When the
on-disk .kicad_pcb has been modified externally between our LoadBoard and
SaveBoard (KiCad GUI's own save, git checkout, another process), the
in-memory state silently overwrites those external changes - losing data
the user can't see was at risk.
This change records the file's mtime_ns + sha256 at LoadBoard and verifies
the signature matches before each auto-save:
* If the signature has diverged, refuse the save and attach a structured
warning to the command result so callers know their mutation is
in-memory only and they need to reload before retrying.
* If it matches, copy the existing file to .mcp-backups/<name>.<ts>
(rotating, keeps last 20) before overwriting.
* Update the recorded signature after our own writes so subsequent
saves are not falsely flagged.
Backwards compatible:
* No tool schemas changed.
* Successful saves return as before, with an extra `autoSave` field
when the wrapper observed something noteworthy.
* Refused saves return success: true (the in-memory mutation did
succeed) plus warnings: [...] and autoSave.diskChangedExternally,
so callers can detect the situation programmatically.
Adds tests/test_auto_save_guard.py (10 tests, all passing) covering:
signature math, refusal on external change, backup creation + content,
backup rotation, first-save semantics (no recorded signature proceeds
normally), and skip cases (no board / no path).
Motivation: the aircam-pdb fork-user lost ~480 traces and the full
footprint layout to a silent overwrite incident on 2026-05-03; recovery
was only possible because VS Code's local-history extension happened to
have a snapshot from a few minutes earlier. This guard makes that class
of incident loud and locally recoverable.
