refactor(joinir): Phase 190 convert.rs modularization

- Created joinir_function_converter.rs (~133 lines): Function-level conversion
- Created joinir_block_converter.rs (~691 lines): Block-level conversion
- Reduced convert.rs from 943 → 120 lines (87% reduction)
- Total: 944 lines (original 943 lines, minimal overhead)
- Separation of concerns: Function vs Block responsibilities
- All handlers moved to block_converter for better organization
- Maintained backward compatibility with existing API
- Build successful, simple tests passing

docs/development/current/main/phase33-11-exit-phi-bug-analysis.md

@@ -0,0 +1,141 @@
+# Phase 33-11: Exit Block PHI Node Missing Bug Analysis
+
+## Problem Summary
+
+The MIR generated from JoinIR lowering is missing PHI nodes in the exit block, causing "use of undefined value" errors.
+
+## Test Case
+
+File: `apps/tests/loop_min_while.hako`
+
+```nyash
+static box Main {
+  main() {
+    local i = 0
+    loop(i < 3) {
+      print(i)
+      i = i + 1
+    }
+    return 0
+  }
+}
+```
+
+## Error
+
+```
+[ERROR] use of undefined value ValueId(16)
+```
+
+## MIR Dump Analysis
+
+```mir
+bb8:
+    1: ret %16    # %16 is never defined!
+```
+
+bb8 is the exit block, but ValueId(16) is never defined anywhere.
+
+## Root Cause
+
+In `src/mir/builder/control_flow.rs::merge_joinir_mir_blocks()`:
+
+### Step 1: Exit block is created empty (line 618-621)
+```rust
+let exit_block_id = self.block_gen.next();
+// ...
+let exit_block = BasicBlock::new(exit_block_id);  // Empty!
+func.add_block(exit_block);
+```
+
+### Step 2: Return instructions are converted to Jump (line 827-841)
+```rust
+MirInstruction::Return { value } => {
+    if let Some(ret_val) = value {
+        let remapped_val = value_map.get(ret_val).copied().unwrap_or(*ret_val);
+        if debug {
+            eprintln!(
+                "[cf_loop/joinir]   Return({:?}) → Jump to exit",
+                remapped_val
+            );
+        }
+    }
+    MirInstruction::Jump {
+        target: exit_block_id,
+    }
+}
+```
+
+**Problem**: The return value (`remapped_val`) is logged but NOT STORED anywhere!
+
+### Step 3: Exit block stays empty
+The exit block is never populated with:
+- PHI nodes to collect return values
+- Return instruction to return the PHI'd value
+
+## Expected Fix
+
+The exit block should look like:
+
+```mir
+bb8:
+    1: %16 = phi [%9 from bb7], [%2 from bb5], ...
+    1: ret %16
+```
+
+Or simpler, if all functions return the same constant:
+
+```mir
+bb8:
+    1: %16 = const 0
+    1: ret %16
+```
+
+## JoinIR Functions Structure
+
+The Pattern 1 lowerer generates 3 functions:
+
+1. **main()**: Calls loop_step, returns 0
+2. **loop_step()**: Tail recursion OR Jump to k_exit
+3. **k_exit()**: Returns 0
+
+All Return instructions are converted to Jump to bb8 (exit block).
+
+## Solution Strategy
+
+### Option A: Collect Return Values + Generate PHI
+1. While converting Return→Jump, collect all return values
+2. After merging, generate PHI node in exit block
+3. Add Return instruction that returns PHI result
+
+### Option B: Direct Value Propagation
+1. Since Pattern 1 always returns 0, directly emit const 0 in exit block
+2. Simpler but less general
+
+### Option C: Track Return Values in Hash Map
+1. Create `HashMap<BasicBlockId, ValueId>` to track returns
+2. Use as PHI incoming values
+3. Most robust, handles all patterns
+
+## Recommendation
+
+Start with **Option B** (simplest fix for Pattern 1), then generalize to **Option C** for future patterns.
+
+## Implementation Location
+
+File: `src/mir/builder/control_flow.rs`
+Function: `merge_joinir_mir_blocks()`
+Lines: ~827-950
+
+## Test Validation
+
+```bash
+NYASH_DISABLE_PLUGINS=1 ./target/release/hakorune apps/tests/loop_min_while.hako
+```
+
+Expected output:
+```
+0
+1
+2
+```