feat(joinir): Phase 170-4 Structure-based routing opt-in

Add NYASH_JOINIR_STRUCTURE_ONLY=1 environment variable to bypass
function name whitelist and route purely based on loop structure.

When enabled:
- Skips hardcoded function name whitelist (13 entries)
- Routes directly to pattern detection (LoopPatternContext)
- Falls back to legacy LoopBuilder if no pattern matches

This is Phase 1 of structure-based routing migration:
- Phase 1: Opt-in via env flag (this commit) ✅
- Phase 2: Make structure-based default (future)
- Phase 3: Remove whitelist entirely (future)

Verified:
- test_loop_return.hako → RC: 2 ✅
- test_trim_loop.hako → RC: 3 ✅

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

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

src/mir/builder/control_flow/joinir/routing.rs

@@ -35,7 +35,18 @@ impl MirBuilder {
         // Phase 195: Use unified trace
         trace::trace().routing("router", &func_name, "try_cf_loop_joinir called");
 
-        // Phase 49-4 + Phase 80: Multi-target routing
+        // Phase 170-4: Structure-based routing option
+        // When NYASH_JOINIR_STRUCTURE_ONLY=1, skip function name whitelist
+        // and route purely based on loop structure analysis
+        let structure_only = std::env::var("NYASH_JOINIR_STRUCTURE_ONLY")
+            .ok()
+            .as_deref()
+            == Some("1");
+
+        if structure_only {
+            trace::trace().routing("router", &func_name, "Structure-only mode enabled, skipping whitelist");
+        } else {
+            // Phase 49-4 + Phase 80: Multi-target routing (legacy whitelist)
             // - Core ON なら代表2本（print_tokens / ArrayExt.filter）は JoinIR を優先し、失敗したら LoopBuilder へフォールバック
             // - Core OFF では従来通り dev フラグで opt-in
             // Note: Arity does NOT include implicit `me` receiver
@@ -83,6 +94,7 @@ impl MirBuilder {
             if !is_target {
                 return Ok(None);
             }
+        }
 
         // Debug log when routing through JoinIR Frontend
         // Phase 195: Check trace flags directly from JoinLoopTrace