docs: update pg_ivm comparison for IMMEDIATE mode completion

[grove]

Update GAP_PG_IVM_COMPARISON.md to reflect that Transactional IVM Phase 1 + 3 are fully implemented.

Changes

• Executive summary: IMMEDIATE mode marked as implemented (not planned); pg_ivm compat layer noted as postponed; remaining gap narrowed to PG version support only
• Project overview: release 0.1.2 → 0.1.3
• Maintenance model §3: 'Planned' → 'Implemented' with Phase 1–4 status and usage example
• SQL feature summary §4: Maintenance timing winner changed from pg_ivm to pg_trickle (offers both models)
• Known limitations §13: staleness caveat updated to mention IMMEDIATE mode; added recursive CTE restriction; migration scripts deferred to v0.3.0+
• Unique features §15: 24 → 27 items (added IMMEDIATE+deferred, TopK, Diamond consistency)
• pg_ivm gaps §15: Bump docker/build-push-action from 5 to 6 #1 (immediate maintenance) and Bump postgres from 18.1 to 18.2 in /tests #3 (TRUNCATE) marked ✅ Closed
• Use-case fit §16: zero-staleness scenarios now recommend 'Either' instead of pg_ivm only
• Coexistence §17: updated to reflect pg_trickle handles both deferred and immediate use-cases
• Recommendations §18: IMMEDIATE mode + extended SQL marked Done; compat layer marked Postponed
• Conclusion §19: 'two remaining structural gaps' → 'one remaining gap' (PG version support only)
• API §6: Added IMMEDIATE mode example alongside existing DIFFERENTIAL example