db: no test for opening a store whose schema version is newer than the binary expects

[Sohex]

Summary

The schema collapsed back to a single migration (V1) before the v1.0.0 release.
Migration is tested in the forward direction (fresh→V1, plus idempotent reopen),
but nothing covers the case of a binary opening a store whose user_version is
newer than the binary knows about — the realistic situation once the store is
an external-writer contract and a newer external tool has bumped the schema.

Evidence

• musefs-db/src/schema.rs has MIGRATIONS = &[MIGRATION_V1] (latest = V1) and
  migration tests cover fresh→V1 and idempotent reopen.
• musefs-db/tests/schema.rs asserts a fresh DB is at user_version = 1 and
  that reopen does not re-migrate.
• migrate() returns early with success when user_version >= latest
  (schema.rs:212), so a store at a newer user_version (e.g. V2 written by a
  future/third-party tool) is treated as already-migrated and opened — there is
  no "store is newer than this binary" guard, and no test asserts a defined
  outcome for that case.

Risk

A future or third-party schema bump can cause an older binary to silently
misread the store rather than failing with a clear "store is newer than this
binary" error, directly affecting the external-writer contract.