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/merge/mod.rs

@@ -583,13 +583,15 @@ pub(in crate::mir::builder) fn merge_joinir_mir_blocks(
     #[cfg(debug_assertions)]
     {
         if let Some(boundary) = boundary {
-            verify_joinir_contracts(
-                builder.function(),
-                entry_block_remapped,
-                exit_block_id,
-                &loop_header_phi_info,
-                boundary,
-            );
+            if let Some(ref func) = builder.current_function {
+                verify_joinir_contracts(
+                    func,
+                    entry_block_remapped,
+                    exit_block_id,
+                    &loop_header_phi_info,
+                    boundary,
+                );
+            }
             if debug {
                 eprintln!("[cf_loop/joinir] Phase 200-3: Contract verification passed");
             }
@@ -647,7 +649,13 @@ fn verify_loop_header_phis(
 ) {
     // Check 1: Loop variable PHI existence
     if let Some(ref loop_var_name) = boundary.loop_var_name {
-        let header_block_data = &func.blocks[header_block.0];
+        let header_block_data = func.blocks.get(&header_block).unwrap_or_else(|| {
+            panic!(
+                "[JoinIRVerifier] Header block {} not found ({} blocks in func)",
+                header_block,
+                func.blocks.len()
+            )
+        });
         let has_loop_var_phi = header_block_data
             .instructions
             .iter()
@@ -663,7 +671,13 @@ fn verify_loop_header_phis(
 
     // Check 2: Carrier PHI existence
     if !loop_info.carrier_phis.is_empty() {
-        let header_block_data = &func.blocks[header_block.0];
+        let header_block_data = func.blocks.get(&header_block).unwrap_or_else(|| {
+            panic!(
+                "[JoinIRVerifier] Header block {} not found ({} blocks in func)",
+                header_block,
+                func.blocks.len()
+            )
+        });
         let phi_count = header_block_data
             .instructions
             .iter()
@@ -715,7 +729,7 @@ fn verify_exit_line(
     boundary: &JoinInlineBoundary,
 ) {
     // Check 1: Exit block exists
-    if exit_block.0 >= func.blocks.len() {
+    if !func.blocks.contains_key(&exit_block) {
         panic!(
             "[JoinIRVerifier] Exit block {} out of range (func has {} blocks)",
             exit_block.0,