fix: undo after shallow snapshot import

[zxch3n]

Summary

Fixes #1012.

Shallow snapshot imports that need to materialize latest state currently emit their internal checkout as a Checkout event. UndoManager treats checkout as a user-visible version switch, clears its stacks, and sets next_counter to None, so the first local commit after import only reinitializes the undo baseline instead of becoming an undo item.

This change keeps explicit checkout behavior unchanged, but lets the shallow snapshot import materialization path emit that internal checkout as an Import event using the original import origin. That preserves subscriber visibility for the imported state while avoiding the UndoManager reset.

Validation

• cargo test -p loro --test loro_rust_test undo_records_first_local_commit_after_shallow_snapshot_import -- --nocapture
• cargo test -p loro --test loro_rust_test shallow_snapshot -- --nocapture
• cargo test -p loro --test issue -- --nocapture
• cargo test -p loro --test contracts events_subscriptions -- --nocapture
• cargo check -p loro-internal

Note: cargo check still reports the existing loro-common/src/lib.rs unused import warning, unrelated to this change.

[github-actions]

WASM Size Report

• Original size: 3031.64 KB
• Gzipped size: 1000.25 KB
• Brotli size: 702.27 KB