debug(mir): add comprehensive receiver tracing and block overwrite protection

This commit investigates ValueId(21) undefined error in Stage-B compilation.

Changes:
- src/mir/builder/builder_calls.rs:
  - Add NYASH_DEBUG_PARAM_RECEIVER=1 trace for method call receivers
  - Track variable_map lookups and ValueId mismatches
  - Log receiver origin and current_block context

- src/mir/builder/utils.rs:
  - Fix start_new_block() to avoid overwriting existing blocks
  - Check if block exists before calling add_block()
  - Prevents Copy instructions from being lost

- src/mir/function.rs:
  - Add warning log when replacing existing block
  - Helps detect block overwrite issues

- lang/src/mir/builder/ (Hako files):
  - Add debug prints for method lowering paths
  - These were not used (Stage-B uses Rust MIR Builder)
  - Kept for future Hako MIR Builder debugging

Key Discovery:
- Stage-B compilation uses Rust MIR Builder, not Hako MIR Builder
- ValueId(21) is undefined receiver in StageBArgsBox.resolve_src/1
- MIR shows: call_method ParserBox.length() [recv: %21] but %21 has no definition
- Likely caused by LocalSSA Copy emission failure or block overwrite

Next: Fix LocalSSA to ensure receiver Copy is properly emitted and preserved

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

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

src/mir/builder/builder_calls.rs

@@ -699,7 +699,28 @@ impl super::MirBuilder {
         }
 
         // 4. Build object value for remaining cases
-        let object_value = self.build_expression(object)?;
+        let object_value = self.build_expression(object.clone())?;
+
+        // CRITICAL DEBUG: Track receiver ValueId for parameter variables
+        if std::env::var("NYASH_DEBUG_PARAM_RECEIVER").ok().as_deref() == Some("1") {
+            use crate::ast::ASTNode;
+            if let ASTNode::Variable { name, .. } = &object {
+                eprintln!("[DEBUG/param-recv] build_method_call receiver '{}' → ValueId({})",
+                          name, object_value.0);
+                if let Some(origin) = self.value_origin_newbox.get(&object_value) {
+                    eprintln!("[DEBUG/param-recv]   origin: {}", origin);
+                }
+                if let Some(&mapped_id) = self.variable_map.get(name) {
+                    eprintln!("[DEBUG/param-recv]   variable_map['{}'] = ValueId({})", name, mapped_id.0);
+                    if mapped_id != object_value {
+                        eprintln!("[DEBUG/param-recv]   ⚠️ MISMATCH! build_expression returned different ValueId!");
+                    }
+                } else {
+                    eprintln!("[DEBUG/param-recv]   ⚠️ '{}' NOT FOUND in variable_map!", name);
+                }
+                eprintln!("[DEBUG/param-recv]   current_block: {:?}", self.current_block);
+            }
+        }
 
         // 5. Handle TypeOp methods: value.is("Type") / value.as("Type")
         // Note: This was duplicated in original code - now unified!