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/function.rs

@@ -98,6 +98,9 @@ impl MirFunction {
     /// Add a new basic block
     pub fn add_block(&mut self, block: BasicBlock) -> BasicBlockId {
         let id = block.id;
+        if self.blocks.contains_key(&id) && std::env::var("NYASH_LOCAL_SSA_TRACE").ok().as_deref() == Some("1") {
+            eprintln!("[mir-function] replacing existing block {:?}", id);
+        }
         self.blocks.insert(id, block);
         id
     }