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>

apps/tests/joinir_if_select_local.hako

@@ -0,0 +1,14 @@
+static box IfSelectLocalTest {
+    main() {
+        local x
+        local cond
+        cond = 1
+        if cond {
+            x = 100
+        } else {
+            x = 200
+        }
+        print(x)
+        return 0
+    }
+}

apps/tests/joinir_if_select_simple.hako

@@ -0,0 +1,16 @@
+static box IfSelectTest {
+    main() {
+        local result
+        result = me.test(1)
+        print(result)
+        return 0
+    }
+
+    test(cond) {
+        if cond {
+            return 10
+        } else {
+            return 20
+        }
+    }
+}