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/dir_fsync_test.go

@@ -0,0 +1,39 @@
+package wal
+
+import (
+	"os"
+	"strings"
+	"testing"
+)
+
+func TestDirFsyncSuccess(t *testing.T) {
+	dir := t.TempDir()
+	if err := dirFsync(dir); err != nil {
+		t.Errorf("dirFsync on valid dir: %v", err)
+	}
+}
+
+func TestDirFsyncNonExistentDir(t *testing.T) {
+	err := dirFsync("/nonexistent/path/that/should/not/exist")
+	if err == nil {
+		t.Fatal("expected error on non-existent dir")
+	}
+	if !strings.Contains(err.Error(), "open dir") {
+		t.Errorf("error should mention 'open dir', got: %v", err)
+	}
+}
+
+func TestDirFsyncNotADirectory(t *testing.T) {
+	dir := t.TempDir()
+	filePath := dir + "/notadir"
+	if err := os.WriteFile(filePath, []byte("x"), 0o644); err != nil {
+		t.Fatalf("WriteFile: %v", err)
+	}
+	err := dirFsync(filePath)
+	if err == nil {
+		t.Fatal("expected error when fsyncing a file as dir")
+	}
+	if !strings.Contains(err.Error(), "not a directory") {
+		t.Errorf("error should mention 'not a directory', got: %v", err)
+	}
+}