feat(joinir): Phase 185 Strict Mode Semantics Cleanup

Remove redundant strict checks from If lowering (3 → 1 check point):
- mod.rs: Remove 2 strict panic blocks from try_lower_if_to_joinir()
- mod.rs: Comment out unused strict_on variable
- Keep single strict check at caller level (if_form.rs)

This aligns If lowering architecture with Loop lowering:
- Lowerers are thin Result-returning boxes (no policy decisions)
- Strict mode check happens at router/caller level (single source of truth)
- Fail-Fast principle: panic at ONE location when strict=ON

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

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

src/mir/join_ir/lowering/mod.rs

@@ -202,7 +202,8 @@ pub fn try_lower_if_to_joinir(
         return None;
     }
     let core_on = crate::config::env::joinir_core_enabled();
-    let strict_on = crate::config::env::joinir_strict_enabled();
+    // Phase 185: strict check moved to caller (if_form.rs)
+    // let strict_on = crate::config::env::joinir_strict_enabled();
 
     // Phase 33-9.1: Loop専任関数の除外（Loop/If責務分離）
     // Loop lowering対象関数はIf loweringの対象外にすることで、
@@ -248,7 +249,7 @@ pub fn try_lower_if_to_joinir(
             return None;
         }
     }
-    let strict_allowed = strict_on && core_on && is_allowed;
+    // Phase 185: strict_allowed removed (strict check moved to caller: if_form.rs)
 
     if debug_level >= 1 {
         eprintln!(
@@ -286,6 +287,8 @@ pub fn try_lower_if_to_joinir(
         if_select::IfSelectLowerer::new(debug_level)
     };
 
+    // Phase 185: Remove strict checks from lowerer (thin Result-returning box)
+    // Strict mode panic should happen at caller level (if_form.rs), not here
     if !if_select_lowerer.can_lower_to_select(func, block_id) {
         if debug_level >= 1 {
             eprintln!(
@@ -293,12 +296,6 @@ pub fn try_lower_if_to_joinir(
                 func.signature.name
             );
         }
-        if strict_allowed {
-            panic!(
-                "[joinir/if] strict mode: pattern not matched for {}",
-                func.signature.name
-            );
-        }
         return None;
     }
 
@@ -311,13 +308,6 @@ pub fn try_lower_if_to_joinir(
         );
     }
 
-    if result.is_none() && strict_allowed {
-        panic!(
-            "[joinir/if] strict mode: lowering failed for {}",
-            func.signature.name
-        );
-    }
-
     result
 }