feat(migrations)!: align runMemoryMigrations with runMigrations

[TheGreatAxios]

Summary

Stacked on #53; merge that first.

• runMemoryMigrations(config, { schema, ftsLanguage }) takes the same DBConfig and schema as Interchange runMigrations. schema is the host schema holding the tenant and principal tables; the "public". foreign-key references are rewritten to it. Memory's own tables stay in the memory schema.
• There is no ledger: every file is idempotent and replayed on each run, and a new migration drops memory._migrations. Foreign keys and CHECK constraints are added only when missing. A foreign key that points at a different host schema fails the replay. The temporal_class backfill runs only in the replay that adds the column.
• The whole replay runs in one transaction behind pg_advisory_xact_lock, the same approach as @corbits/artifacts, so replicas that boot together run it one at a time instead of racing CREATE ... IF NOT EXISTS or deadlocking. There is no lock_timeout, matching artifacts: a replay waits behind a long reader instead of failing boot, and a failed replay rolls back whole and releases the lock.
• ftsLanguage is required, with no default. The runner no longer reads FTS_LANGUAGE from the environment.
• config is checked by type only: @intx/db 0.4.0 exports DBConfig as a type, not a validator. @intx/db is a required peer because DBConfig is part of the signature.
• CONTRIBUTING states the migration rule: never change a shipped file's effect; widening goes in a new file.
• The README migration snippet is one call.

Verification

• bun run typecheck and bun test ./src pass (457 tests).
• Migrations run twice on fresh databases on a local pgvector Postgres 16 with host schemas public, hub and a quoted name. The foreign keys target <schema>.tenant/principal. A database migrated by the 0.1.0 runner upgrades cleanly and loses its ledger.
• The end-to-end suite in test: add a real-Postgres e2e suite with add and search tests #56 tests the idempotent replay, the host schema, waiting behind a long reader, and three concurrent runners.

Closes CL-9138
Closes CL-9143
Closes CL-9180

[TheGreatAxios]

Self-review, four passes over the stacked range.

Verdict: ready for review.

• Replays are idempotent on real Postgres, including a database migrated by the old runner.
• The host-schema rewrite matches Interchange's regex, and a foreign key into a different host schema fails the replay.
• Waivers and the type-only DBConfig note are in the description.