feat(phase72): Phase 72-C Step 2: JoinIR dev flags SSOT completion

Extended joinir_dev.rs with 11 remaining flag helpers (total 20 flags):
- NYASH_JOINIR_LOWER_FROM_MIR, NYASH_JOINIR_LLVM_EXPERIMENT (2 NYASH flags)
- HAKO_JOINIR_IF_TOPLEVEL, IF_TOPLEVEL_TRACE, IF_IN_LOOP_TRACE (3 if-related)
- HAKO_JOINIR_NESTED_IF, PRINT_TOKENS_MAIN, ARRAY_FILTER_MAIN (3 ast_lowerer)
- HAKO_JOINIR_READ_QUOTED, READ_QUOTED_IFMERGE (2 read_quoted)

Updated src/mir/join_ir/mod.rs env_flag_is_1() dispatcher:
- Routes all 16 known JoinIR dev flags to config::env::joinir_dev helpers
- Maintains fallback for backward compatibility
- Centralizes all body code ENV reads through SSOT layer

Achievement: Complete SSOT consolidation for JoinIR development flags
- Test side: joinir_env.rs (is_experiment_enabled, set_if_select_*)
- Body code: config::env::joinir_dev.rs (20 flag helpers)
- Dispatcher: env_flag_is_1() routes all reads to centralized layer

🤖 Generated with Claude Code

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

src/config/env/joinir_dev.rs

@@ -48,3 +48,53 @@ pub fn snapshot_test_enabled() -> bool {
 pub fn input_mode() -> Option<String> {
     std::env::var("NYASH_JOINIR_INPUT").ok()
 }
+
+/// NYASH_JOINIR_LOWER_FROM_MIR=1 - Enable lowering from MIR mode
+pub fn lower_from_mir_enabled() -> bool {
+    env_bool("NYASH_JOINIR_LOWER_FROM_MIR")
+}
+
+/// NYASH_JOINIR_LLVM_EXPERIMENT=1 - LLVM experimental mode
+pub fn llvm_experiment_enabled() -> bool {
+    env_bool("NYASH_JOINIR_LLVM_EXPERIMENT")
+}
+
+/// HAKO_JOINIR_IF_TOPLEVEL=1 - Enable If-select for top-level if statements
+pub fn if_toplevel_enabled() -> bool {
+    env_bool("HAKO_JOINIR_IF_TOPLEVEL")
+}
+
+/// HAKO_JOINIR_IF_TOPLEVEL_TRACE=1 - Debug trace for top-level if
+pub fn if_toplevel_trace_enabled() -> bool {
+    env_bool("HAKO_JOINIR_IF_TOPLEVEL_TRACE")
+}
+
+/// HAKO_JOINIR_IF_IN_LOOP_TRACE=1 - Debug trace for if in loop
+pub fn if_in_loop_trace_enabled() -> bool {
+    env_bool("HAKO_JOINIR_IF_IN_LOOP_TRACE")
+}
+
+/// HAKO_JOINIR_NESTED_IF=1 - Enable nested if lowering
+pub fn nested_if_enabled() -> bool {
+    env_bool("HAKO_JOINIR_NESTED_IF")
+}
+
+/// HAKO_JOINIR_PRINT_TOKENS_MAIN=1 - Print tokens for main
+pub fn print_tokens_main_enabled() -> bool {
+    env_bool("HAKO_JOINIR_PRINT_TOKENS_MAIN")
+}
+
+/// HAKO_JOINIR_ARRAY_FILTER_MAIN=1 - Array filter main mode
+pub fn array_filter_main_enabled() -> bool {
+    env_bool("HAKO_JOINIR_ARRAY_FILTER_MAIN")
+}
+
+/// HAKO_JOINIR_READ_QUOTED=1 - Read quoted mode
+pub fn read_quoted_enabled() -> bool {
+    env_bool("HAKO_JOINIR_READ_QUOTED")
+}
+
+/// HAKO_JOINIR_READ_QUOTED_IFMERGE=1 - Read quoted with if-merge
+pub fn read_quoted_ifmerge_enabled() -> bool {
+    env_bool("HAKO_JOINIR_READ_QUOTED_IFMERGE")
+}