feat: add core upgrade migration path

[dangelo352]

Summary

• add commitment_core version storage with get_version, admin-only upgrade, and admin-only migrate entrypoints
• reject all-zero WASM hashes and invalid/replayed migrations
• backfill legacy counters/indexes/guard without changing commitment state
• document the commitment_core upgrade and migration path

Fixes #465

Tests

• CARGO_TARGET_DIR=/tmp/commitlabs-core-upgrade-target cargo test -p commitment_core migrate -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-upgrade-target cargo test -p commitment_core upgrade -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-upgrade-target cargo test -p commitment_core settle -- --nocapture
• CARGO_TARGET_DIR=/tmp/commitlabs-core-upgrade-target cargo test -p commitment_core early_exit -- --nocapture

Note: I also ran cargo test -p commitment_core create_commitment -- --nocapture; it exercised the create suite but hit two pre-existing event/id expectation failures unrelated to this migration change.