feat: Make create_constraint enforce index uniqueness

[Ludv1gL]

Summary

Addresses review feedback on #4465. The previous implementation only inserted metadata into system tables but never converted the in-memory index from non-unique to unique.

• Index infrastructure: Add SameKeyEntry::count(), iter_duplicates(), check_and_into_unique() on MultiMap/HashIndex for duplicate detection via existing index infra. Add from_non_unique()/into_non_unique() for lossless conversion between unique and non-unique index types. Add TypedIndex::make_unique()/make_non_unique()/iter_duplicates() via define_uniqueness_conversions! macro covering all 36 variant pairs.
• Table: Add take_pointer_map()/restore_pointer_map()/has_unique_index() for pointer map lifecycle management.
• Datastore: Rename create_constraint → create_st_constraint (metadata-only, used by create_table). New create_constraint calls create_st_constraint then makes index unique on both tx and commit tables with can_merge check. Same pattern for drop_constraint/drop_st_constraint. Enrich PendingSchemaChange::ConstraintAdded with IndexId and PointerMap for correct rollback.
• Migration: Remove CheckAddUniqueConstraintValid precheck — duplicate detection now happens inside create_constraint using the index directly. Remove dead AddUniqueConstraint error variant.
• Tests: 6 transactionality tests covering create/drop constraint for single-col, multi-col, rollback, and duplicate rejection.

Test plan

• cargo test -p spacetimedb-table — 99 tests pass
• cargo test -p spacetimedb-datastore --features test — 89 tests pass (6 new)
• cargo test -p spacetimedb-schema -- auto_migrate — 12 tests pass
• cargo build -p spacetimedb-core — compiles clean

🤖 Generated with Claude Code

[bfops]

Thank you for opening a PR! If this is meant to respond to feedback on #4465, then the commits should be pushed to that PR since it won't be approved + merged with unaddressed feedback.

[Ludv1gL]

Superseded — pushed these changes to #4465 instead.