feat(wal): implement batch codec, validation, and Arena allocator

- wal/batch.go: WalBatch encode/decode with FragmentCollector state machine
- wal/validate.go: ValidateBatchLimits with checked arithmetic
- memtable/arena.go: Arena allocator with 8-byte alignment and mutex
- 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": "2026-06-12T05:22:47.301Z",
+      "updated_at": "2026-06-12T05:26:58.955Z",
       "session_ids": [
         "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
       ],
@@ -42,6 +42,19 @@
           "status": "completed",
           "ended_at": "2026-06-12T05:22:47.301Z",
           "elapsed_ms": 173304
+        },
+        "todo:6": {
+          "task_key": "todo:6",
+          "task_label": "6",
+          "task_title": "WAL Batch encode/decode + fragment collector",
+          "session_id": "opencode:ses_145b65d08ffebmSv4sk5I69JXZ",
+          "agent": "Sisyphus-Junior",
+          "category": "unspecified-high",
+          "updated_at": "2026-06-12T05:26:58.955Z",
+          "started_at": "2026-06-12T05:26:43.370Z",
+          "status": "completed",
+          "ended_at": "2026-06-12T05:26:58.955Z",
+          "elapsed_ms": 15585
         }
       }
     }
@@ -49,7 +62,7 @@
   "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": "2026-06-12T05:22:47.301Z",
+  "updated_at": "2026-06-12T05:26:58.955Z",
   "session_ids": [
     "opencode:ses_145c3bae9ffeTB2zbsTym0Cev8"
   ],
@@ -83,6 +96,19 @@
       "status": "completed",
       "ended_at": "2026-06-12T05:22:47.301Z",
       "elapsed_ms": 173304
+    },
+    "todo:6": {
+      "task_key": "todo:6",
+      "task_label": "6",
+      "task_title": "WAL Batch encode/decode + fragment collector",
+      "session_id": "opencode:ses_145b65d08ffebmSv4sk5I69JXZ",
+      "agent": "Sisyphus-Junior",
+      "category": "unspecified-high",
+      "updated_at": "2026-06-12T05:26:58.955Z",
+      "started_at": "2026-06-12T05:26:43.370Z",
+      "status": "completed",
+      "ended_at": "2026-06-12T05:26:58.955Z",
+      "elapsed_ms": 15585
     }
   },
   "agent": "atlas"