fix: Code review round 2 — race conditions, dead code, hardcoded paths

CRITICAL fixes:
- #1 vm.go: Mutex on libModules/dynamicFuncs global slices
  RegisterLibModule/RegisterDynamicFunc now thread-safe
  RegisterLibModules copies under lock, clears, releases
- #4 shutdown.go: Signal handler goroutine leak fixed
  Uses done channel + select for clean exit on normal shutdown

HIGH fixes:
- #7-8 gobridge.go: Remove dead if/else branches (both identical)
- #13-14 main.go: Remove hardcoded /mnt/d/harbour-core paths
  Use HB_INC env var + standard /usr/local/include/harbour only
- #15 main.go: Remove unused frbModSeq variable

MEDIUM fixes:
- #22 expr.go: Remove unused parts variable in parseInterpolatedString
- #51 macro.go: Remove var _ = fmt.Sprintf import guard
- macroeval.go: Remove unused lexer import and guard

Total fixed this session: 12/53 issues resolved
Remaining: 41 (CRITICAL: 1, HIGH: 9, MEDIUM: 16, LOW: 16)

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

docs/.pdca-status.json

@@ -1,6 +1,6 @@
 {
   "version": "2.0",
-  "lastUpdated": "2026-04-01T01:16:33.700Z",
+  "lastUpdated": "2026-04-01T01:30:25.114Z",
   "activeFeatures": [
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     "hbrtl",
@@ -33,9 +33,9 @@
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       "timestamps": {
         "started": "2026-03-27T09:33:04.512Z",
-        "lastUpdated": "2026-04-01T01:14:37.127Z"
+        "lastUpdated": "2026-04-01T01:30:25.114Z"
       },
-      "lastFile": "/mnt/d/charles/five/hbrt/goroutine.go"
+      "lastFile": "/mnt/d/charles/five/hbrt/macroeval.go"
     },
     "hbrtl": {
       "phase": "do",
@@ -111,7 +111,7 @@
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       "timestamps": {
         "started": "2026-03-27T11:38:35.393Z",
-        "lastUpdated": "2026-03-30T23:12:37.002Z"
+        "lastUpdated": "2026-04-01T01:28:45.794Z"
       },
       "lastFile": "/mnt/d/charles/five/compiler/parser/expr.go"
     },
@@ -137,7 +137,7 @@
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       "timestamps": {
         "started": "2026-03-27T11:50:31.420Z",
-        "lastUpdated": "2026-03-30T23:06:13.211Z"
+        "lastUpdated": "2026-04-01T01:27:55.511Z"
       },
       "lastFile": "/mnt/d/charles/five/cmd/five/main.go"
     },
@@ -280,7 +280,7 @@
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-    "lastActivity": "2026-04-01T01:16:33.700Z"
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@@ -5484,6 +5484,90 @@
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 }