Event tables: datastore tests, migration validation, and bootstrap fix

[cloutiertyler]

Description of Changes

Cherry-picks the core event table implementation (fa83d6d40, 07d57a552) and adds:

1. Bootstrap fix: Register st_event_table in bootstrap_system_tables() — it was missing from the cherry-picked commit, causing event table creation to fail at runtime when inserting into ST_EVENT_TABLE_ID.

2. 4 datastore unit tests covering event table behaviors:

   • Insert+delete in the same tx cancels out (no TxData entry, no committed state)
   • Update yields only the final row in TxData (old row replaced, not accumulated)
   • Bare insert records in TxData but not in committed state
   • replay_insert is a no-op for event tables (commitlog replay doesn't rebuild state)

3. Migration validation: ChangeTableEventFlag error in auto_migrate.rs prevents changing a table's event flag (event→non-event or vice versa) via auto-migration. Includes tests for both directions and a positive test that identical flags are accepted.

4. Test fixture updates: Bootstrap test expectations updated for the new st_event_table system table (table row, column, index ID 23, constraint ID 19). Enabled spacetimedb-schema/test feature in datastore dev-deps to fix pre-existing test compilation issue with for_test methods.

API and ABI breaking changes

None. The ChangeTableEventFlag error variant is added to the internal AutoMigrateError enum but this is not a public API.

Expected complexity level and risk

1 — Straightforward additions. The bootstrap fix is a one-liner that was simply missing from the cherry-pick. The tests exercise existing behavior without changing it. The migration guard follows the exact same pattern as the existing ChangeTableType check.

Testing

• cargo test -p spacetimedb-datastore — 77 tests pass (73 existing + 4 new)
• cargo test -p spacetimedb-schema --features test --lib — 98 tests pass (96 existing + 2 new)
• cargo check --workspace --exclude view-client — compiles clean (view-client has a pre-existing issue with CanBeLookupTable from the cherry-pick)
• Reviewer: confirm that the 4 event table test names/assertions match the behaviors described in review feedback

[cloutiertyler]

[dev-dependencies]
spacetimedb-schema = { path = "../schema", features = ["test"] }

This enables the test feature of spacetimedb-schema when running datastore tests. The spacetimedb-schema crate is already a regular dependency of the datastore crate — my change just adds it as a dev-dependency
with the test feature flag enabled.

The test feature gates for_test() constructors on types like TableName, Identifier, and ColumnSchema. The datastore tests use these constructors extensively (e.g., TableName::for_test("Foo") in TableRow
conversions), but the feature wasn't being enabled in test builds — so the tests were already broken on master before our changes.

In short: the datastore already depends on schema. I'm just enabling the test feature so the existing test helpers compile.

I am not sure why claude is insistent on this. Could someone with knowledge verify that this is true and necessary?