feat(phase32): L-2.1 Stage-1 UsingResolver JoinIR integration + cleanup

Phase 32 L-2.1 complete implementation:

1. Stage-1 UsingResolver main line JoinIR connection
   - CFG-based LoopForm construction for resolve_for_source/5
   - LoopToJoinLowerer integration with handwritten fallback
   - JSON snapshot tests 6/6 PASS

2. JoinIR/VM Bridge improvements
   - Simplified join_ir_vm_bridge.rs dispatch logic
   - Enhanced json.rs serialization
   - PHI core boxes cleanup (local_scope_inspector, loop_exit_liveness, loop_var_classifier)

3. Stage-1 CLI enhancements
   - Extended args.rs, groups.rs, mod.rs for new options
   - Improved stage1_bridge module (args, env, mod)
   - Updated stage1_cli.hako

4. MIR builder cleanup
   - Simplified if_form.rs control flow
   - Removed dead code from loop_builder.rs
   - Enhanced phi_merge.rs

5. Runner module updates
   - json_v0_bridge/lowering.rs improvements
   - dispatch.rs, selfhost.rs, modes/vm.rs cleanup

6. Documentation updates
   - CURRENT_TASK.md, AGENTS.md
   - Various docs/ updates

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

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

src/mir/builder/if_form.rs

@@ -1,5 +1,5 @@
 use super::{MirBuilder, ValueId};
-use crate::ast::{ASTNode, BinaryOperator};
+use crate::ast::ASTNode;
 use crate::mir::loop_api::LoopBuilderApi; // for current_block()
 
 impl MirBuilder {
@@ -13,35 +13,7 @@ impl MirBuilder {
     ) -> Result<ValueId, String> {
         // Reserve a deterministic join id for debug region labeling
         let join_id = self.debug_next_join_id();
-        // Heuristic pre-pin: if condition is a comparison, evaluate its operands now and pin them
-        // so that subsequent branches can safely reuse these values across blocks.
-        // This leverages existing variable_map merges (PHI) at the merge block.
-        if crate::config::env::mir_pre_pin_compare_operands() {
-            if let ASTNode::BinaryOp {
-                operator,
-                left,
-                right,
-                ..
-            } = &condition
-            {
-                match operator {
-                    BinaryOperator::Equal
-                    | BinaryOperator::NotEqual
-                    | BinaryOperator::Less
-                    | BinaryOperator::LessEqual
-                    | BinaryOperator::Greater
-                    | BinaryOperator::GreaterEqual => {
-                        if let Ok(lhs_v) = self.build_expression((**left).clone()) {
-                            let _ = self.pin_to_slot(lhs_v, "@if_lhs");
-                        }
-                        if let Ok(rhs_v) = self.build_expression((**right).clone()) {
-                            let _ = self.pin_to_slot(rhs_v, "@if_rhs");
-                        }
-                    }
-                    _ => {}
-                }
-            }
-        }
+        // Pre-pin heuristic was deprecated; keep operands as-is for predictability.
 
         let condition_val = self.build_expression(condition)?;
         let condition_val = self.local_cond(condition_val);

src/mir/builder/phi_merge.rs

@@ -110,6 +110,7 @@ impl<'a> PhiMergeHelper<'a> {
     ///
     /// # Returns
     /// Ok(()) on success, Err(String) on failure
+    #[allow(dead_code)] // Reserved: explicit dst PHI merge for future use
     pub fn merge_with_dst(
         &mut self,
         dst: ValueId,