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

BUG_AUDIT.md

@@ -105,11 +105,11 @@
 
 ## 🔄 Оставащи задачи (всички non-critical)
 
-1. **Build warnings cleanup** — implicit `cstring` conversion (wal.nim), `HoleEnumConv` (server.nim), unused `os` import (logging.nim)
+1. ~~**Build warnings cleanup**~~ ✅ — implicit `cstring` conversion (wal.nim), `HoleEnumConv` (server.nim), unused `os` import (logging.nim), `ImplicitDefaultValue` (ast.nim)
 2. **Threadpool deprecation** — миграция към `malebolgia`/`weave`/`taskpools`
 3. **Auth SCRAM-SHA-256** — истински challenge-response със salt + iteration count
-4. **TLA+ symmetry reduction** — `SYMMETRY` в `.cfg` файловете за 3-10x по-бърз TLC
-5. **`crossmodal.tla`** — cross-modal consistency между document/vector/graph/FTS
+4. ~~**TLA+ symmetry reduction**~~ ✅ — `SYMMETRY` добавен във всички 9 `.cfg` файла + `Permutations` дефиниции в `.tla` спековете
+5. ~~**`crossmodal.tla`**~~ ✅ — cross-modal consistency между document/vector/graph/FTS
 6. **Replication data transfer** — `writeLsn` да изпраща данни към replicas
 7. **Sharding data migration** — `rebalance` да мигрира ключове
 8. **Property-based / fuzz tests** — storage engine edge cases