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

src/barabadb/storage/wal.nim

@@ -78,14 +78,14 @@ proc writeCommit*(wal: var WriteAheadLog, timestamp: uint64) =
 
 proc sync*(wal: var WriteAheadLog) =
   wal.stream.flush()
-  let fd = posix.open(wal.path, O_RDONLY)
+  let fd = posix.open(cstring(wal.path), O_RDONLY)
   if fd != -1:
     discard posix.fsync(fd)
     discard posix.close(fd)
 
 proc close*(wal: var WriteAheadLog) =
   wal.stream.flush()
-  let fd = posix.open(wal.path, O_RDONLY)
+  let fd = posix.open(cstring(wal.path), O_RDONLY)
   if fd != -1:
     discard posix.fsync(fd)
     discard posix.close(fd)