Phase 33-2: JoinInst::Select implementation + minimal If JoinIR lowering

Implementation:
- Add JoinInst::Select variant to JoinIR schema
- Implement Select execution in JoinIR Runner (Bool/Int cond support)
- Add Select handling in JoinIR→MIR Bridge (4-block structure)
- Create test cases (joinir_if_select_simple/local.hako)
- Add dev toggle NYASH_JOINIR_IF_SELECT=1
- Create lowering infrastructure (if_select.rs, stub for Phase 33-3)

Tests:
- 3/3 unit tests pass (test_select_true/false/int_cond)
- Integration tests pass (RC: 0)
- A/B execution verified (existing if_phi vs JoinIR Select)

Files changed:
- New: apps/tests/joinir_if_select_{simple,local}.hako
- New: src/mir/join_ir/lowering/if_select.rs
- Modified: src/mir/join_ir/{mod,json,runner,vm_bridge}.rs
- Modified: src/config/env.rs (joinir_if_select_enabled)
- Modified: docs/reference/environment-variables.md

Phase 33-3 ready: MIR pattern recognition + auto-lowering pending

🤖 Generated with Claude Code
Co-Authored-By: Claude <noreply@anthropic.com>

src/mir/join_ir/json.rs

@@ -133,6 +133,15 @@ fn write_inst<W: Write>(inst: &JoinInst, out: &mut W) -> std::io::Result<()> {
             }
             write!(out, "}}")?;
         }
+        // Phase 33: Select instruction JSON serialization
+        JoinInst::Select { dst, cond, then_val, else_val } => {
+            write!(out, "{{\"type\":\"select\"")?;
+            write!(out, ",\"dst\":{}", dst.0)?;
+            write!(out, ",\"cond\":{}", cond.0)?;
+            write!(out, ",\"then_val\":{}", then_val.0)?;
+            write!(out, ",\"else_val\":{}", else_val.0)?;
+            write!(out, "}}")?;
+        }
         JoinInst::Compute(mir_like) => {
             write!(out, "{{\"type\":\"compute\",\"op\":")?;
             write_mir_like_inst(mir_like, out)?;