feat(wal,memtable): implement segment writer, block writer, and MemTable

- wal/block_writer.go: 32KB block buffer with padding and flush
- wal/segment_writer.go: WAL segment file with durable-ready protocol
- memtable/memtable.go: Arena+SkipList wrapper with publish/abort semantics
- Comprehensive tests for all modules, all pass with -race

.omo/boulder.json

@@ -8,7 +8,7 @@
       "plan_name": "phase1-wal",
       "status": "active",
       "started_at": "2026-06-12T05:09:32.588Z",
-      "updated_at": "2026-06-12T05:33:52.854Z",
+      "updated_at": "2026-06-12T05:43:11.052Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -68,6 +68,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T05:33:52.854Z",
           "elapsed_ms": 64624
+        },
+        "todo:10": {
+          "task_key": "todo:10",
+          "task_label": "10",
+          "task_title": "Segment writer (file write + block writer)",
+          "session_id": "opencode:ses_145acc017ffe2fra67VT2L28LQ",
+          "agent": "Sisyphus-Junior",
+          "category": "unspecified-high",
+          "updated_at": "2026-06-12T05:43:11.052Z",
+          "started_at": "2026-06-12T05:42:38.284Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T05:43:11.052Z",
+          "elapsed_ms": 32768
         }
       }
     }
@@ -75,7 +88,7 @@
   "active_plan": "/home/dailz/workspace/src/go-kv/.omo/plans/phase1-wal.md",
   "started_at": "2026-06-12T05:09:32.588Z",
   "status": "active",
-  "updated_at": "2026-06-12T05:33:52.854Z",
+  "updated_at": "2026-06-12T05:43:11.052Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -135,6 +148,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T05:33:52.854Z",
       "elapsed_ms": 64624
+    },
+    "todo:10": {
+      "task_key": "todo:10",
+      "task_label": "10",
+      "task_title": "Segment writer (file write + block writer)",
+      "session_id": "opencode:ses_145acc017ffe2fra67VT2L28LQ",
+      "agent": "Sisyphus-Junior",
+      "category": "unspecified-high",
+      "updated_at": "2026-06-12T05:43:11.052Z",
+      "started_at": "2026-06-12T05:42:38.284Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T05:43:11.052Z",
+      "elapsed_ms": 32768
     }
   },
   "agent": "atlas"