P0: route context-installed Internal ID allocator through SQLite mutations #56

Tracked by teaql/teaql-conformance#92 (EXT-ID-INTERNAL). On current main@3f289aa, UserContext.with_internal_id_generator stores a generator and context.generate_id/next_id can call it, but the SQL INSERT path in src/teaql/sql/executor.py allocates via self.next_id and reserves explicit floors via self.ensure_id_floor; no SQL mutation calls context.generate_id. This makes the installed context capability ineffective for persisted INSERTs and risks a silent fallback to teaql_id_space.

Design one typed context-owned async allocator interface with next_id(entity) and ensure_id_floor(entity, floor), independent of the data service. Route direct INSERT, nested batch, explicit ID and graph saves through it, retaining the provider default only when no override is installed. Reject malformed installations before provider access; propagate allocator failures without fallback. Preserve transaction/uniqueness semantics and allow gaps after failure.

Acceptance: focused SQLite positive, concurrent, floor, failure/provider-not-called tests; runnable local-source example; exact-main CI; then immutable internal artifact consumer. No public release or generator adoption in this issue.