feat: MemRDD — in-memory database engine (Go map-based)

Complete in-memory RDD implementation:
- CRUD: Create, Append, GetValue, PutValue, Delete, Recall
- Navigation: GoTo, GoTop, GoBottom, Skip, BOF, EOF
- Index: CreateIndex (sorted slice + binary search), Seek (exact + soft)
- Bulk: Pack (remove deleted), Zap (clear all)
- Multi-open: shared table across work areas
- Driver registered as "MEMRDD", prefix "mem:"

Tests: 9 tests including 5000-record stress test
  Create, Append/Get/Put, Navigation, Delete/Pack, Zap,
  Index (string + numeric), Seek (exact + soft), Stress 5000, Multi-open

Use cases: temp tables, query results, pivot, caching
Performance: no disk I/O, no byte packing — pure Go slices

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/.pdca-status.json

@@ -1,6 +1,6 @@
 {
   "version": "2.0",
-  "lastUpdated": "2026-03-31T01:34:48.246Z",
+  "lastUpdated": "2026-03-31T02:56:59.267Z",
   "activeFeatures": [
     "hbrt",
     "hbrtl",
@@ -19,7 +19,8 @@
     "examples",
     "tmp",
     "genpc",
-    "analyzer"
+    "analyzer",
+    "mem"
   ],
   "primaryFeature": "hbrt",
   "features": {
@@ -256,6 +257,19 @@
         "lastUpdated": "2026-03-31T01:34:48.246Z"
       },
       "lastFile": "/mnt/d/charles/five/compiler/analyzer/analyzer.go"
+    },
+    "mem": {
+      "phase": "do",
+      "phaseNumber": 3,
+      "matchRate": null,
+      "iterationCount": 0,
+      "requirements": [],
+      "documents": {},
+      "timestamps": {
+        "started": "2026-03-31T02:54:31.551Z",
+        "lastUpdated": "2026-03-31T02:56:59.267Z"
+      },
+      "lastFile": "/mnt/d/charles/five/hbrdd/mem/memrdd_test.go"
     }
   },
   "pipeline": {
@@ -266,7 +280,7 @@
   "session": {
     "startedAt": "2026-03-27T06:06:49.620Z",
     "onboardingCompleted": false,
-    "lastActivity": "2026-03-31T01:34:48.246Z"
+    "lastActivity": "2026-03-31T02:56:59.267Z"
   },
   "history": [
     {
@@ -5386,6 +5400,24 @@
       "feature": "analyzer",
       "phase": "do",
       "action": "updated"
+    },
+    {
+      "timestamp": "2026-03-31T02:54:31.551Z",
+      "feature": "mem",
+      "phase": "do",
+      "action": "updated"
+    },
+    {
+      "timestamp": "2026-03-31T02:55:37.952Z",
+      "feature": "mem",
+      "phase": "do",
+      "action": "updated"
+    },
+    {
+      "timestamp": "2026-03-31T02:56:59.267Z",
+      "feature": "mem",
+      "phase": "do",
+      "action": "updated"
     }
   ]
 }