feat: embed dal.DB so Database satisfies the sealed interface

[trakhimenok]

Summary

• dal.DB (dal-go/dalgo v0.64.2) is now sealed by an unexported marker method, produced only by dal.NewDB. Database used to name its dalgo2sql delegate as a plain field (innerDB dal.DB), which cannot promote that marker method, so *Database stopped satisfying dal.DB. Fixed by embedding dal.DB instead of naming it.
• database_dal.go's pure forwarding methods move from d.innerDB.X to d.DB.X.
• The write methods (Set/SetMulti/Insert/Upsert/Delete/DeleteMulti/Update/UpdateMulti) need more than a rename: dalgo2sql's backend does not implement dal.WriteSession in full at the database level (no database-level InsertMulti or UpdateRecord), so dal.NewDB never wraps it in the validating write pipeline there — d.DB's dynamic type has none of these methods at all, verified empirically. dal.As recovers dalgo2sql's concrete backend, which does implement them directly. This is unchanged behaviour: these direct, non-transactional writes were never validated even before this change, since dalgo2sql's database-level write path has no BeforeSave call of its own. Writes made through RunReadwriteTransaction are validated, because that goes through d.DB's own RunReadwriteTransaction.
• Embedding dal.DB alongside dal.ConcurrencyAvailable makes SupportsConcurrentConnections an ambiguous selector (both declare it at the same promotion depth), so Database now defines it explicitly.
• Bumps github.com/dal-go/dalgo to v0.64.2 and github.com/dal-go/dalgo2sql to v0.10.0 (the already-converted release these adapters build on).
• Adds conformance_test.go wiring dalgotest.RunConformance, gated behind this package's existing DALGO2POSTGRES_TEST_DSN env var, matching every other DB-backed test here.

Known gap (not fixed here, reporting per audit)

There is no dal.BackendOf-equivalent for a transaction handed into a RunReadwriteTransaction worker, so adapter-specific transaction methods are unreachable from inside a worker. This package doesn't define its own transaction type (it delegates entirely to dalgo2sql's), so it does not hit this gap itself, but it is a real upstream limitation worth tracking.

Test plan

• gofmt -l . — clean
• GOWORK=off go vet ./... — clean
• GOWORK=off go test -race -count=1 ./... — ok, but 15 of 16 tests skip (no DALGO2POSTGRES_TEST_DSN in this environment); only TestNewDatabase_RejectsBadDSN actually runs and passes (no live server needed). No live PostgreSQL server was available to me, so TestConformance could not be exercised for real — it also skips.
• Run the full suite (including TestConformance) against a live PostgreSQL server with DALGO2POSTGRES_TEST_DSN set, to actually exercise the conformance suite.

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https://claude.ai/code/session_01SkkrXdtf8mU2GRo2hHsHT1