feat(joinir): Phase 183 LoopBodyLocal role separation + test fixes

Phase 183 Implementation:
- Added is_var_used_in_condition() helper for AST variable detection
- Implemented LoopBodyLocal filtering in TrimLoopLowerer
- Created 4 test files for P1/P2 patterns
- Added 5 unit tests for variable detection

Test Fixes:
- Fixed test_is_outer_scope_variable_pinned (BasicBlockId import)
- Fixed test_pattern2_accepts_loop_param_only (literal node usage)

Refactoring:
- Unified pattern detection documentation
- Consolidated CarrierInfo initialization
- Documented LoopScopeShape construction paths

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

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

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

@@ -39,10 +39,16 @@ impl MirBuilder {
         // 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")
+        // Phase 196: Default to structure-first routing now that LoopBuilder is removed.
+        // - Default: ON (structure_only = true) to allow JoinIR patterns to run for all loops.
+        // - To revert to the previous whitelist-only behavior, set NYASH_JOINIR_STRUCTURE_ONLY=0.
+        let structure_only = match std::env::var("NYASH_JOINIR_STRUCTURE_ONLY")
             .ok()
             .as_deref()
-            == Some("1");
+        {
+            Some("0") | Some("off") => false,
+            _ => true,
+        };
 
         if structure_only {
             trace::trace().routing("router", &func_name, "Structure-only mode enabled, skipping whitelist");