feat(phase73): Selfhost Stage-B I/O 簡素化 - HAKO_SRC 環境変数統一化

Unified selfhost Stage-B I/O path from argv to environment variables:
- Remove --source "$SRC_CONTENT" from compiler.hako invocation (argv limit)
- Export HAKO_SRC="$SRC_CONTENT" instead (env-based I/O)
- Update compiler_stageb.hako to check HAKO_SRC fallback

Changes:
1. lang/src/compiler/entry/compiler_stageb.hako:
   - Added HAKO_SRC env check in StageBArgsBox.resolve_src()
   - Fallback order: --source arg → HAKO_SOURCE_FILE_CONTENT → HAKO_SRC → default

2. tools/selfhost/selfhost_build.sh:
   - Removed --source "$SRC_CONTENT" from compiler.hako invocation
   - Export HAKO_SRC="$SRC_CONTENT" before Stage-B execution
   - Unified with wrapper pathway (HAKO_USE_BUILDBOX=1)

Benefits:
- Eliminates "Argument list too long" (os error 7) for large .hako files
- Single unified I/O pattern (env variables) for all Stage-B paths
- Consistent with hako_check.sh and wrapper pattern
- Ready for future HAKO_SRC_FILE optimization

Verification:
✅ NYASH_FEATURES=stage3 NYASH_USE_NY_COMPILER=1 \
  ./tools/selfhost/selfhost_build.sh --in apps/tests/stage1_run_min.hako --run
  Output: abc (correct)

🤖 Generated with Claude Code

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

tools/selfhost/selfhost_build.sh

@@ -93,10 +93,11 @@ else
   apply_selfhost_env
   stageb_cmd_desc="compiler.hako --stage-b --stage3"
   (
+    export HAKO_SRC="$SRC_CONTENT"
     cd "$ROOT" && \
       "$BIN" --backend vm \
         "$ROOT/lang/src/compiler/entry/compiler.hako" -- \
-        --stage-b --stage3 --source "$SRC_CONTENT"
+        --stage-b --stage3
   ) > "$RAW" 2>&1 || stageb_rc=$?
 fi