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_manager_test.go

@@ -1,8 +1,10 @@
 package wal
 
 import (
+	"errors"
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 
 	"github.com/dailz/go-kv/config"
@@ -239,3 +241,67 @@ func itoa(n uint64) string {
 	}
 	return string(buf[i:])
 }
+
+// Regression guard for C6: NewSegmentManager must propagate dir fsync
+// failure from initial segment creation. This is the DB.Open failure path.
+func TestNewSegmentManagerFailsOnDirFsyncFailure(t *testing.T) {
+	dir := t.TempDir()
+	cfg := tinyWalConfig()
+
+	orig := dirFsyncFn
+	dirFsyncFn = func(string) error { return errors.New("simulated dir fsync failure") }
+	t.Cleanup(func() { dirFsyncFn = orig })
+
+	sm, err := NewSegmentManager(dir, 0, 0, cfg)
+	if err == nil {
+		if sm != nil {
+			sm.Close()
+		}
+		t.Fatal("NewSegmentManager: expected error on dir fsync failure, got nil")
+	}
+	if !strings.Contains(err.Error(), "create initial segment") {
+		t.Errorf("error should be wrapped as 'create initial segment', got: %v", err)
+	}
+}
+
+// Regression guard for C6: SegmentManager.AppendBatch must propagate
+// rotation failure (which now includes dir fsync failure) as error.
+//
+// Note: C8 (segment_manager.go:64-66 passes byte offset as startSequence)
+// makes multi-segment recovery broken, but this test only verifies error
+// propagation through AppendBatch; it does not exercise recovery.
+func TestSegmentManagerRotateFailsOnDirFsyncFailure(t *testing.T) {
+	dir := t.TempDir()
+	cfg := tinyWalConfig()
+
+	sm, err := NewSegmentManager(dir, 0, 0, cfg)
+	if err != nil {
+		t.Fatalf("NewSegmentManager: %v", err)
+	}
+	defer sm.Close()
+
+	encoded, err := EncodeWalBatch(0, []*WalEntry{
+		{OpType: OpPut, ValueKind: VKInline, Key: []byte("k"), Value: []byte("v")},
+	})
+	if err != nil {
+		t.Fatalf("EncodeWalBatch: %v", err)
+	}
+
+	// Fill the active segment until next AppendBatch would trigger rotation.
+	// segment_manager.go:62 triggers rotate when
+	//   RemainingPayload() < len(encoded) + 2*PhysicalRecordHeaderSize
+	worstCaseSize := uint64(len(encoded)) + 2*uint64(PhysicalRecordHeaderSize)
+	for sm.RemainingPayload() >= worstCaseSize {
+		if err := sm.AppendBatch(encoded); err != nil {
+			t.Fatalf("fill AppendBatch: %v", err)
+		}
+	}
+
+	orig := dirFsyncFn
+	dirFsyncFn = func(string) error { return errors.New("simulated dir fsync failure") }
+	t.Cleanup(func() { dirFsyncFn = orig })
+
+	if err := sm.AppendBatch(encoded); err == nil {
+		t.Fatal("AppendBatch: expected rotation failure, got nil")
+	}
+}