fix(emit): stabilize Stage-B wrapper with temp file approach

Root Cause:
- Subshell CODE expansion became path literal "/cat/tmp/matmul.hako"
- Variable lost in nested subshell with cd command
- All benchmark cases (matmul, arraymap, etc.) failed emit

Solution:
- Temp file approach with trap cleanup (CODE_TMP=$(mktemp))
- 3-tier fallback extraction (Python→awk→ruby)
- Enhanced diagnostics with HAKO_SELFHOST_TRACE=1
- Pre-check SKIP logic in microbench for unstable emit

Changes:
- tools/hakorune_emit_mir.sh
  - Temp file approach eliminates subshell variable issues
  - extract_program_json now has 3 fallback strategies
  - Detailed trace output for debugging
  - Variable scope fixes (local → script level)
- tools/perf/microbench.sh
  - matmul pre-check with SKIP + diagnostic hint
  - Prevents false benchmark results on emit failure

Test Results:
✅ loop:        936 bytes   rc=0
✅ call:        330 bytes   rc=0
✅ stringchain: 313 bytes   rc=0
✅ arraymap:    422 bytes   rc=0
✅ matmul:     7731 bytes   rc=0 (FIXED!)
✅ CI guard: emit_provider_no_jsonfrag_canary PASS

Impact:
- All benchmark cases now emit MIR successfully
- Stable execution without subshell variable bugs
- Comprehensive diagnostics for future debugging
- Foundation for provider-first optimization

Next: Apply AotPrep to optimize Array/Map hot paths

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

lang/src/compiler/entry/compiler_stageb.hako

@@ -22,15 +22,23 @@ static box Main {
   main(args) {
     // 1) Collect source from args or env
     local src = null
+    local src_file = null
     if args != null {
       local i = 0
       local n = args.length()
       loop(i < n) {
         local t = "" + args.get(i)
         if t == "--source" && i + 1 < n { src = "" + args.get(i + 1)  break }
+        if t == "--source-file" && i + 1 < n { src_file = "" + args.get(i + 1)  i = i + 1 }
         i = i + 1
       }
     }
+    // Prefer explicit source string when provided
+    // If --source-file is used, prefer env-provided content (wrapper supplies HAKO_SOURCE_FILE_CONTENT)
+    if src == null && src_file != null {
+      local inline = env.get("HAKO_SOURCE_FILE_CONTENT")
+      if inline != null { src = "" + inline }
+    }
     // Skip env.local.get check - Stage-3 keyword conflict
     // Original: if src == null { src = env.local.get("HAKO_SOURCE") }
     if src == null { src = "return 0" }