feat(selfhost): Add Stage-3 LOCAL keyword support to ParserStmtBox

🔧 Problem: Selfhost ParserBox doesn't recognize LOCAL keyword
   - Only supports lowercase 'local' (Stage-1/2)
   - Stage-3 mode with HAKO_PARSER_STAGE3=1 not working

✅ Solution: Add LOCAL keyword recognition in ParserStmtBox
   - Check ctx.stage3_enabled() before parsing
   - Support both 'local' and 'LOCAL' keywords
   - Maintain backward compatibility with Stage-1/2

📍 Modified: lang/src/compiler/parser/stmt/parser_stmt_box.hako:109-149
   - Added is_local_kw flag and kw_len variable
   - Check 'local' first (always)
   - Check 'LOCAL' if stage3_enabled() (conditional)

🐛 Current status:
   - Using-chain parsing: ✅ SUCCESS (2 files)
   - Stage-B test: ❌ Still failing with 'Unexpected token LOCAL at line 19'
   - Next: Investigate which file's line 19 is causing the error

Related: #stageb-緑化 #phase-20.33 #selfhost-parser

lang/src/compiler/parser/stmt/parser_stmt_box.hako

@@ -106,9 +106,22 @@ static box ParserStmtBox {
       return "{\"type\":\"Return\",\"expr\":" + expr_json_ret + "}"
     }
 
-    // local declaration
+    // local declaration (Stage-3 compatible: supports both 'local' and 'LOCAL')
+    local is_local_kw = 0
+    local kw_len = 0
     if ctx.starts_with_kw(src, j, "local") == 1 {
-      j = j + 5
+      is_local_kw = 1
+      kw_len = 5
+    }
+    if is_local_kw == 0 && ctx.stage3_enabled() == 1 {
+      if ctx.starts_with_kw(src, j, "LOCAL") == 1 {
+        is_local_kw = 1
+        kw_len = 5
+      }
+    }
+
+    if is_local_kw == 1 {
+      j = j + kw_len
       j = ctx.skip_ws(src, j)
       local idp = ctx.read_ident2(src, j)
       local at = idp.lastIndexOf("@")