fix: make WAL segment directory fsync failure fatal (C6)

Per design §3.2 line 248-272, segment directory fsync is a hard
requirement for durable-ready state, not best-effort. rename is atomic
in memory but not guaranteed to survive power loss without a directory
fsync. The previous code silently swallowed both os.Open(dir) and
dirFD.Sync() errors, leaving WAL writer to confirm batches as durable
when their segment might not exist after a crash.

Failure propagation:
- Initial segment creation: NewSegmentWriter fails -> NewSegmentManager
  fails -> DB.Open fails (user sees error, no data promise violated).
- Rotation during AppendBatch: NewSegmentWriter fails -> AppendBatch
  fails -> WalWriter.stopWithError(ErrCommitUnknown) -> write-stopped
  (per design line 272).

Changes:
- wal/segment_writer.go: extract dirFsync helper (Open -> f.Stat ->
  IsDir -> f.Sync, avoiding TOCTOU window), replace silent swallow with
  fatal error; on failure clean up resources (fd.Close + os.Remove) and
  surface cleanup errors via errors.Join so nothing is silently lost.
- wal/dir_fsync_test.go (new): unit test the helper with valid dir,
  non-existent dir (fails at os.Open), and not-a-dir (fails at IsDir).
- wal/segment_writer_test.go: add TestNewSegmentWriterDirFsyncFailure
  (injects failure via package-level dirFsyncFn override; documents the
  not-parallel-safe constraint), TestNewSegmentWriterNormalPathStillWorks
  (regression), and TestNewSegmentWriterRetryAfterDirFsyncFailure
  (verifies cleanup is effective for retry).
- wal/segment_manager_test.go: add TestSegmentManagerRotateFailsOnDirFsyncFailure
  (fills segment until rotation triggers, injects failure, verifies
  propagation through AppendBatch path) and TestNewSegmentManagerFailsOnDirFsyncFailure
  (covers the DB.Open failure path).

dirFsyncFn injection note: tests that override this package-level var
must not use t.Parallel(). All existing wal tests run serially within
the package; this is the lightest mechanism that doesn't require
interface indirection in production code.

Verified: each new test fails on pre-fix code (silent swallow returned
nil error) and passes after the fix. Full suite green including
go test -race ./... .

Audit context: docs/audit-3.2.md C6 (Oracle-verified bg_ef425776).

wal/segment_writer.go

@@ -1,6 +1,7 @@
 package wal
 
 import (
+	"errors"
 	"fmt"
 	"os"
 	"path/filepath"
@@ -82,10 +83,18 @@ func NewSegmentWriter(
 		return nil, fmt.Errorf("wal: open segment file for append: %w", err)
 	}
 
-	// Sync directory to make rename durable (best-effort on Linux).
-	if dirFD, derr := os.Open(dir); derr == nil {
-		dirFD.Sync()
-		dirFD.Close()
+	// Per design §3.2 line 258, directory fsync is a hard requirement for
+	// durable-ready. Without it, the rename above is not guaranteed to survive
+	// power loss, violating the Always-mode "no loss of acknowledged writes"
+	// promise.
+	if err := dirFsyncFn(dir); err != nil {
+		closeErr := fd.Close()
+		removeErr := os.Remove(finalPath)
+		if closeErr != nil || removeErr != nil {
+			cleanup := errors.Join(closeErr, removeErr)
+			return nil, fmt.Errorf("wal: fsync directory after segment rename (cleanup: %v): %w", cleanup, err)
+		}
+		return nil, fmt.Errorf("wal: fsync directory after segment rename: %w", err)
 	}
 
 	return &SegmentWriter{