SQLite store - V1 schema implementation.

Every public defn in this file is dispatched dynamically by
`vis-sdk.core/defdelegate` via `ns-resolve`; clj-kondo never sees
the call sites. The ns-level config above silences
`:unused-public-var` for the whole file. The actual call surface
is verified through the storage facade tests.

Tables (V1__schema.sql):
  session_soul, session_state,
  session_turn_soul, session_turn_state,
  session_turn_iteration, llm_routing_event,
  extension_aggregate,
  log

Connection lifecycle:
  (db-open! db-spec)   -> {:datasource ds :path ...}
  (db-close! store)    -> idempotent dispose

Space reclamation for a file-backed SQLite store.

SQLite never hands the file back on its own: `auto_vacuum` is off, so pages a
delete or a `DROP COLUMN` frees go on the FREELIST and are only reused by
later writes. That is the right default — reuse costs nothing — but a one-off
bulk reclaim (retiring a column across a whole transcript) leaves hundreds of
megabytes the file keeps until someone runs `VACUUM`.

`maybe-vacuum!` is that someone, on the same fortnight window as
`foundation.housekeeping` retention: at most once per `vacuum-interval-days`
per store, and only when the freelist is worth the rewrite — at least
`vacuum-min-free-bytes` AND `vacuum-min-free-fraction` of the file. A compact
store is never rewritten, so the usual answer is three PRAGMAs and no I/O.

VACUUM takes SQLite's exclusive lock and rewrites the whole file (measured:
15 s and 2.4 GB -> 2.0 GB on a real store), so it runs OFF the open path —
`vacuum-async!` on a lowest-priority daemon thread after a settling delay,
never inside a transaction. Readers and writers in this process or another
one block for its duration and then continue, which the 30 s busy timeout and
the write-retry ladder in `core` absorb. A process that exits first, or a
`db-close!` that aborts the lease mid-rewrite, simply leaves the store due at
the next start: the rewrite is transactional, so an interrupted VACUUM rolls
back rather than damaging anything.

The `vis.db.vacuum` marker beside the store is BOTH the clock (its mtime is
the last successful vacuum) and the cross-process mutex (an exclusive
`FileLock` held for the rewrite). It is created only when a vacuum is
actually attempted and deleted again when one fails, so a store that never
needed reclaiming carries no marker and stays due.

Flyway-backed schema migration runner.

Lives in the SQLite extension because:

  1. The dialect-specific Flyway driver (`flyway-database-nc-sqlite`)
     is required to recognize `jdbc:sqlite:` URLs - already
     declared in this extension's deps.edn.
  2. Flyway is the only backend-side concern using
     `flyway-core`; making it a per-backend dep keeps the root
     package free of the migration toolchain.
  3. The previous arrangement shipped a generic `migrate!` from
     `com.blockether.vis.sdk`, but it had exactly one caller
     - this extension. Other backends will ship their own
     migration entry point in their own jar.

Public API:

  `(migrate! datasource locations)` - apply every Flyway
  migration found at the given classpath `locations` to the
  supplied `DataSource`. Returns the datasource for thread-style
  chaining.

`:baseline-on-migrate true` so existing databases without a
`flyway_schema_history` table get one on first run. `:mixed true`
so SQL files with mixed transactional + DDL statements work under
SQLite.

GraalVM native-image note: Flyway discovers migrations by LISTING the
classpath location directory, which native-image does not support (it can
`getResource` a specific file but not enumerate a dir). So `build.clj` writes
an `_index.edn` of filenames next to each migration dir, and here we feed
Flyway an explicit `ResourceProvider` built from those exact paths. On the
JVM (no index) we fall back to Flyway's normal location scanning.