fix(joinir): Pattern 4 PHI loss fix with exit_bindings and block reuse

Two root causes for Pattern 4 outputting 0 instead of 25:

1. instruction_rewriter.rs:92 - BasicBlock::new() was overwriting
   existing blocks that already contained PHI instructions from
   JoinIR Select lowering. Fixed by removing and reusing existing
   blocks instead of creating new ones.

2. pattern4_with_continue.rs - JoinIR exit PHI (ValueId 15) was not
   connected to host's sum variable, causing DCE to eliminate the PHI
   as "unused". Fixed by using new_with_exit_bindings() with explicit
   LoopExitBinding to connect k_exit's sum_exit to host's sum slot.

Test result: loop_continue_pattern4.hako now correctly outputs 25.

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

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

src/mir/builder/control_flow/joinir/merge/instruction_rewriter.rs

@@ -89,7 +89,15 @@ pub(super) fn merge_and_rewrite(
             let new_block_id = remapper
                 .get_block(func_name, *old_block_id)
                 .ok_or_else(|| format!("Block {:?} not found for {}", old_block_id, func_name))?;
-            let mut new_block = BasicBlock::new(new_block_id);
+
+            // Phase 195 FIX: Reuse existing block if present (preserves PHI from JoinIR Select lowering)
+            // ultrathink "finalizer集約案": Don't overwrite blocks with BasicBlock::new()
+            let mut new_block = if let Some(ref mut current_func) = builder.current_function {
+                current_func.blocks.remove(&new_block_id)
+                    .unwrap_or_else(|| BasicBlock::new(new_block_id))
+            } else {
+                BasicBlock::new(new_block_id)
+            };
 
             // Phase 189 FIX: Check if this is entry function's entry block (for boundary input skipping)
             let is_entry_func_entry_block =