fix: snapshot() returns deep copies to prevent internal store mutation

[Copilot]

snapshot() returned shallow dict(v) copies, meaning callers could mutate nested structures (e.g. chapters_done, consistency) and corrupt the internal store — inconsistent with the deep-copy guarantee already provided by get().

Changes

• novelforge/progress.py: Replace dict(v) with copy.deepcopy(v) in snapshot(); update docstring to reflect deep-copy semantics.
• tests/test_progress_manager.py: Rename test_snapshot_returns_shallow_copies → test_snapshot_returns_deep_copies and add a nested-mutation assertion that directly exercises the fixed behaviour.

# Before
def snapshot(self) -> dict[str, ProgressState]:
    with self._lock:
        return {k: dict(v) for k, v in self._store.items()}

# After
def snapshot(self) -> dict[str, ProgressState]:
    with self._lock:
        return {k: copy.deepcopy(v) for k, v in self._store.items()}

[Copilot]

Pull request overview

This PR aligns ProgressManager.snapshot() with get() by returning deep copies, preventing callers from mutating nested structures and corrupting the internal in-memory progress store.

Changes:

• Update ProgressManager.snapshot() to return copy.deepcopy(...) entries instead of shallow dict(...) copies.
• Update the snapshot() docstring to reflect deep-copy semantics and safe caller mutation.
• Strengthen the snapshot test to assert nested mutations don’t affect stored state.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
novelforge/progress.py	Make snapshot() return deep copies and update documentation to match the intended immutability boundary.
tests/test_progress_manager.py	Rename and extend the snapshot test to cover nested-mutation behavior.

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[Copilot]

snapshot() now uses copy.deepcopy, which should generally preserve the ProgressState type. Consider removing the broad # type: ignore[misc] (or replacing it with a narrower cast(...) / correct ignore code) so mypy can validate the return type instead of suppressing it.