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/utils.rs

@@ -70,7 +70,9 @@ impl super::MirBuilder {
     /// Start a new basic block and set as current
     pub(crate) fn start_new_block(&mut self, block_id: BasicBlockId) -> Result<(), String> {
         if let Some(ref mut function) = self.current_function {
-            function.add_block(BasicBlock::new(block_id));
+            if !function.blocks.contains_key(&block_id) {
+                function.add_block(BasicBlock::new(block_id));
+            }
             self.current_block = Some(block_id);
             // Local SSA cache is per-block; clear on block switch
             self.local_ssa_map.clear();