malkhut: DuckDB file-backed asset store — schema, sync, queries, persistence

DuckDB store (asset_store.py):
- 4 tables: assets (28 cols), exchanges (10 cols), asset_exchanges (junction),
  behavior_profiles (28 cols)
- Schema with indexes on blockchain, coingecko_id, cmc_id, exchange_id
- sync_from_profiles(): populate from in-memory dicts in one call
- Query API: query_assets(**filters), get_asset(), symbols_for_exchange(),
  assets_on_blockchain(), asset_count(), exchange_count()
- Case-insensitive exchange lookup
- File-backed persistence: data survives connection close/reopen
- Performance: <1s sync, 100 queries in <1s, 1000 gets in <1s

30 tests covering:
- Schema creation and table structure (3 tests)
- Sync from profiles (6 tests, idempotent)
- Asset CRUD (7 tests, query by blockchain/coingecko/sector)
- Exchange mapping (5 tests, case-insensitive)
- Behavior profiles (4 tests)
- Persistence across connections (2 tests)
- Performance baseline (3 tests)

Total: 1276 tests across 50 files, all green, zero regressions.

MALKHUT/malkhut/storage/__init__.py

@@ -1 +1,3 @@
-from malkhut.storage.ch_store import MalkhutCHStore
+from malkhut.storage.asset_store import AssetStore
+
+__all__ = ["AssetStore"]