feat(joinir): Phase 245C - Function parameter capture + test fix

Extend CapturedEnv to include function parameters used in loop conditions,
enabling ExprLowerer to resolve variables like `s` in `loop(p < s.length())`.

Phase 245C changes:
- function_scope_capture.rs: Add collect_names_in_loop_parts() helper
- function_scope_capture.rs: Extend analyze_captured_vars_v2() with param capture logic
- function_scope_capture.rs: Add 4 new comprehensive tests

Test fix:
- expr_lowerer/ast_support.rs: Accept all MethodCall nodes for syntax support
  (validation happens during lowering in MethodCallLowerer)

Problem solved: "Variable not found: s" errors in loop conditions

Test results: 924/924 PASS (+13 from baseline 911)

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/mir/join_ir/lowering/scope_manager.rs

@@ -24,7 +24,6 @@ use super::condition_env::ConditionEnv;
 use super::loop_body_local_env::LoopBodyLocalEnv;
 use super::carrier_info::CarrierInfo;
 use crate::mir::loop_pattern_detection::function_scope_capture::CapturedEnv;
-use std::collections::BTreeMap;
 
 /// Phase 231: Scope kind for variables
 ///