fix(joinir): Phase 283 P0 - Pattern3 Undefined ValueId Bug Fix

- Fix: extract_if_condition() moved after local_cond_env construction
  (loop_with_if_phi_if_sum.rs:175)
  - Root cause: condition extraction before i_param/sum_param creation
  - Result: i % 2 referenced caller's ConditionEnv with unmapped ValueId
- Fail-Fast: Add condition_bindings validation in merge (mod.rs)
- Fixture: Update loop_if_phi.hako for C2 compatibility (sum.toString())
- Verified: VM execution outputs sum=9 ✅

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

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

docs/development/current/main/phases/phase-283/README.md

@@ -0,0 +1,47 @@
+# Phase 283: JoinIR If-Condition ValueId Remap Fix
+
+Status: **P0 ✅ complete (2025-12-23)**
+
+Goal:
+- Fix a Pattern3 (if-sum) JoinIR lowering bug where complex if-conditions like `i % 2 == 1` could produce MIR that uses an undefined `ValueId`.
+
+SSOT References:
+- JoinIR lowering (if-sum): `src/mir/join_ir/lowering/loop_with_if_phi_if_sum.rs`
+- Condition value lowering: `src/mir/join_ir/lowering/condition_lowerer.rs`
+- MIR verification (undefined values): `src/mir/verification/ssa.rs`
+- Test fixture: `apps/tests/loop_if_phi.hako`
+
+## Problem
+
+Observed error (VM):
+- `use of undefined value ValueId(...)` inside the loop body, originating from the modulo expression in the if-condition.
+
+The symptom appeared in `apps/tests/loop_if_phi.hako`:
+```hako
+if (i % 2 == 1) { ... } else { ... }
+```
+
+## Root Cause
+
+In `lower_if_sum_pattern`, the if-condition was extracted (and its JoinIR value-expression lowered) **before** `i_param` / `sum_param` existed, so `Variable("i")` inside complex expressions resolved through the caller-provided `ConditionEnv`.
+
+That early `ConditionEnv` mapping could not be remapped by the boundary/merger, resulting in JoinIR ValueIds that became undefined host ValueIds in MIR.
+
+## Fix (P0)
+
+- Move `extract_if_condition(...)` so it runs **after** `local_cond_env` is constructed and shadowed:
+  - `local_cond_env.insert(loop_var, i_param)`
+  - `local_cond_env.insert(update_var, sum_param)`
+  - (and any condition bindings remapped to `main_params`)
+
+This ensures complex expressions like `i % 2` resolve `i` to `i_param` (a loop_step param that the merger remaps), preventing undefined ValueIds.
+
+## Fixture Note (Coercion SSOT)
+
+After Phase 275 (C2), implicit `"String" + Integer` is a TypeError. The fixture uses:
+- `sum.toString()` instead of `"sum=" + sum` or `str(sum)`.
+
+## Acceptance
+
+- `apps/tests/loop_if_phi.hako` runs on VM without undefined ValueId errors and prints `sum=9`.
+