feat: clean build + crossmodal.tla + TLA+ symmetry (v1.0.0-prep)

Build warnings cleanup (0 warnings):
- Suppress threadpool deprecation warning (baradadb.nim)
- Remove unused 'os' import (logging.nim)
- Fix ImplicitDefaultValue for newNode params (ast.nim)
- Add explicit cstring cast for posix.open (wal.nim)
- Fix HoleEnumConv for MsgKind parsing (server.nim)

TLA+ Formal Verification:
- Add symmetry reduction (Permutations) to all 9 existing specs
- Add SYMMETRY directive to all .cfg files
- New crossmodal.tla: cross-modal consistency spec
  * MetadataVectorConsistency, HybridResultValid
  * CommittedAtomicity, AbortedAtomicity, TxnStateValid
- New models/crossmodal.cfg

Tests:
- Add Cross-Modal TLA+ Faithfulness tests (4 tests)

Docs:
- Update PLAN.md and BUG_AUDIT.md with completed tasks

formal-verification/replication.tla

@@ -157,4 +157,7 @@ TypeOk ==
   /\ appliedLsn \in 0..MaxLsn
   /\ ackedBy \in [0..MaxLsn -> SUBSET Replicas]
 
+\* Symmetry reduction for model checking.
+Symmetry == Permutations({r1, r2, r3})
+
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