feat(stageb): Phase 25.1c - Stage-B トレース追加（dev-only）

追加内容:
- StageBTraceBox: dev トレース用 Box 追加（HAKO_STAGEB_TRACE=1 で有効）
- トレースポイント:
  - StageBArgsBox.resolve_src: enter/return_len
  - StageBBodyExtractorBox.build_body_src: enter_len/return_len
  - StageBDriverBox.main: enter/after_resolve_src/after_build_body_src/
    after_parse_program2/func_scan methods/exit rc=0

Phase 25.1c 目標:
- Stage-B / Stage-1 CLI 構造デバッグ
- fib canary / selfhost CLI canary の rc=1 原因特定

ポリシー:
- dev env でガード（挙動不変）
- 既定挙動は変更せず、観測のみ追加

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

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

lang/src/compiler/entry/compiler_stageb.hako

@@ -17,9 +17,28 @@ using lang.compiler.entry.func_scanner as FuncScannerBox
 using lang.compiler.entry.using_resolver as Stage1UsingResolverBox
 using lang.compiler.builder.mod as CompilerBuilder
 
+// Dev-only trace helper (Phase 25.1c)
+// - Enabled when HAKO_STAGEB_TRACE=1
+// - Keeps Stage-B behavior unchanged（ログのみ追加）
+static box StageBTraceBox {
+  log(label) {
+    local flag = env.get("HAKO_STAGEB_TRACE")
+    if flag == null { return 0 }
+    if ("" + flag) != "1" { return 0 }
+    // label が null/Void でも落ちないように守る（dev専用）
+    local msg = "[stageb/trace]"
+    if label != null {
+      msg = msg + " " + ("" + label)
+    }
+    print(msg)
+    return 0
+  }
+}
+
 // Phase 25.1c: CLI argument → source resolution
 static box StageBArgsBox {
   resolve_src(args) {
+    StageBTraceBox.log("StageBArgsBox.resolve_src:enter")
     // 1) Collect source from args or env
     local src = null
     local src_file = null
@@ -43,6 +62,13 @@ static box StageBArgsBox {
     // Original: if src == null { src = env.local.get("HAKO_SOURCE") }
     if src == null { src = "return 0" }
 
+    {
+      // Trace final source length（dev専用）
+      local l = 0
+      if src != null { l = ("" + src).length() }
+      StageBTraceBox.log("StageBArgsBox.resolve_src:return_len=" + ("" + l))
+    }
+
     return src
   }
 }
@@ -50,6 +76,12 @@ static box StageBArgsBox {
 // Phase 25.1c: Body extraction + bundle + using + trim
 static box StageBBodyExtractorBox {
   build_body_src(src, args) {
+    {
+      //入口トレース: 入力ソース長と引数有無
+      local l = 0
+      if src != null { l = ("" + src).length() }
+      StageBTraceBox.log("StageBBodyExtractorBox.build_body_src:enter len=" + ("" + l))
+    }
     // ============================================================================
     // Depth guard: prevent accidental recursion inside Stage‑B body extractor
     // ============================================================================
@@ -609,6 +641,13 @@ static box StageBBodyExtractorBox {
       if e > b { body_src = s.substring(b, e) } else { body_src = "" }
     }
 
+    {
+      //出口トレース: 抽出後 body_src 長
+      local l2 = 0
+      if body_src != null { l2 = ("" + body_src).length() }
+      StageBTraceBox.log("StageBBodyExtractorBox.build_body_src:return_len=" + ("" + l2))
+    }
+
     // Clear depth guard before returning
     env.set("HAKO_STAGEB_BODY_DEPTH", "0")
     return body_src
@@ -630,7 +669,14 @@ static box StageBDriverBox {
       env.set("HAKO_STAGEB_DRIVER_DEPTH", "1")
     }
 
+    StageBTraceBox.log("StageBDriverBox.main:enter")
+
     local src = StageBArgsBox.resolve_src(args)
+    {
+      local l = 0
+      if src != null { l = ("" + src).length() }
+      StageBTraceBox.log("StageBDriverBox.main:after_resolve_src len=" + ("" + l))
+    }
 
     // 2) Stage‑3 acceptance default ON for selfhost (env may turn off; keep tolerant here)
     local p = new ParserBox()
@@ -643,11 +689,22 @@ static box StageBDriverBox {
     // local externs_json = p.get_externs_json()
 
     local body_src = StageBBodyExtractorBox.build_body_src(src, args)
+    {
+      local l2 = 0
+      if body_src != null { l2 = ("" + body_src).length() }
+      StageBTraceBox.log("StageBDriverBox.main:after_build_body_src len=" + ("" + l2))
+    }
 
     // 6) Parse and emit Stage‑1 JSON v0 (Program)
     // Bridge(JSON v0) が Program v0 を受け取り MIR に lowering するため、ここでは AST(JSON v0) を出力する。
     // 既定で MIR 直出力は行わない（重い経路を避け、一行出力を保証）。
     local ast_json = p.parse_program2(body_src)
+    {
+      // AST(JSON v0) の長さを軽く観測
+      local la = 0
+      if ast_json != null { la = ("" + ast_json).length() }
+      StageBTraceBox.log("StageBDriverBox.main:after_parse_program2 len=" + ("" + la))
+    }
 
     // 6.3) Apply SSA transformations (CompilerBuilder pipeline)
     {
@@ -686,6 +743,12 @@ static box StageBDriverBox {
         // Use FuncScannerBox to extract method definitions from all boxes
         local methods = FuncScannerBox.scan_all_boxes(src)
 
+        {
+          local cnt = 0
+          if methods != null { cnt = methods.length() }
+          StageBTraceBox.log("StageBDriverBox.main:func_scan methods=" + ("" + cnt))
+        }
+
         // Build defs JSON array
         if methods.length() > 0 {
           defs_json = ",\"defs\":["
@@ -734,6 +797,7 @@ static box StageBDriverBox {
     }
 
     print(ast_json)
+    StageBTraceBox.log("StageBDriverBox.main:exit rc=0")
     // Clear depth guard before returning
     env.set("HAKO_STAGEB_DRIVER_DEPTH", "0")
     return 0