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/builder/control_flow/debug.rs

@@ -23,6 +23,7 @@ impl MirBuilder {
     ///
     /// Phase 195: Delegates to JoinLoopTrace for unified tracing.
     /// Enable with NYASH_TRACE_VARMAP=1
+    #[allow(dead_code)]
     pub(in crate::mir::builder) fn trace_varmap(&self, context: &str) {
         super::joinir::trace::trace().varmap(context, &self.variable_map);
     }