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/backend/mir_interpreter/handlers/calls/method.rs

@@ -405,10 +405,10 @@ impl MirInterpreter {
             // Plugin Box methods (slot >= 1000)
             (_, slot) if slot >= 1000 => {
                 if let VMValue::BoxRef(bx) = receiver {
-                    if let Some(p) = bx.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>() {
+                    if let Some(_p) = bx.as_any().downcast_ref::<crate::runtime::plugin_loader_v2::PluginBoxV2>() {
                         let host = crate::runtime::plugin_loader_unified::get_global_plugin_host();
-                        let host = host.read().unwrap();
-                        let argv = self.load_args_as_boxes(args)?;
+                        let _host = host.read().unwrap();
+                        let _argv = self.load_args_as_boxes(args)?;
                         // Get method name from slot (reverse lookup would be needed in production)
                         // For now, fall back to old path
                         return Err(self.err_with_context(
@@ -474,7 +474,7 @@ impl MirInterpreter {
             // StringBox special methods (is_space, is_alpha)
             if box_ref.type_name() == "StringBox" {
                 let s_box = box_ref.to_string_box();
-                let s = s_box.value;
+                let _s = s_box.value;
                 match method {
                     "is_space" => {
                         if let Some(arg_id) = args.get(0) {