Adds engine/importer.py exposing commit_import / get_undo_entry /
undo_import. The committer applies a classified adds/replaces/renames
payload inside a single sqlite transaction, dispatching by `kind` to
the modules table (5 module types), bundles, or module_categories.
Module writes recompute snapshot_fingerprint; bundle writes recompute
payload_hash; categories use case-insensitive name-based merge
(matching the legacy import_bundle handler).
Every successful commit persists an import_undo row with enough
metadata (imported_records, replaced_snapshots, rename_map) to
reverse the commit. undo_import restores replaced rows bit-for-bit
(preserving original version + payload_hash + timestamps) and
deletes inserted rows, then evicts the undo entry.
Migration 007 creates the import_undo metadata table. Adds
17 importer tests covering adds across all 7 buckets, replace
snapshotting, rename with old→new mapping, collision rollback,
and undo restore/delete paths.
