feat(phase72-c): JoinIR dev flags SSOT foundation - config::env helpers

Phase 72-C: Created centralized SSOT helpers for all 9 DevOnly JoinIR flags.

New module: src/config/env/joinir_dev.rs
- lower_generic_enabled() → NYASH_JOINIR_LOWER_GENERIC (CRITICAL: 15 uses)
- mainline_debug_enabled() → NYASH_JOINIR_MAINLINE_DEBUG (5 uses)
- if_merge_enabled() → NYASH_JOINIR_IF_MERGE
- debug_enabled() → NYASH_JOINIR_DEBUG (deprecated)
- vm_bridge_enabled() → NYASH_JOINIR_VM_BRIDGE
- strict_enabled() → NYASH_JOINIR_STRICT
- snapshot_generate_enabled() → NYASH_JOINIR_SNAPSHOT_GENERATE
- snapshot_test_enabled() → NYASH_JOINIR_SNAPSHOT_TEST
- input_mode() → NYASH_JOINIR_INPUT

Benefits:
- All ENV reads routed through single module (joinir_dev)
- Easy to extend with validation, logging, or tracing
- Consistent naming: *_enabled() for bool flags, input_mode() for string value
- Foundation for next step: replace direct std::env calls in code

Next steps:
1. Update joinir_env test helpers with similar pattern
2. Replace std::env::var("NYASH_JOINIR_*") calls with config::env::joinir_dev::*
3. Update tests to use centralized helpers

Tests verified: cargo check passed with no errors

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

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

src/config/env.rs

@@ -5,10 +5,12 @@
 
 mod catalog;
 pub mod dump;
+pub mod joinir_dev;
 pub mod stage1;
 
 pub use catalog::{env_vars, AppliesTo, EnvVarMeta};
 pub use dump::*;
+pub use joinir_dev::*;
 pub use stage1::*;
 use std::collections::BTreeMap;
 

src/config/env/joinir_dev.rs

@@ -0,0 +1,50 @@
+//! JoinIR development / experimental flags (SSOT).
+//! Phase 72-C: Consolidate all NYASH_JOINIR_* dev flags through centralized helpers.
+
+use crate::config::env::env_bool;
+
+/// NYASH_JOINIR_LOWER_GENERIC=1 - Enable generic lowering path for JoinIR
+/// (CRITICAL: 15 occurrences in codebase)
+pub fn lower_generic_enabled() -> bool {
+    env_bool("NYASH_JOINIR_LOWER_GENERIC")
+}
+
+/// NYASH_JOINIR_MAINLINE_DEBUG=1 - Debug output for mainline JoinIR lowering
+pub fn mainline_debug_enabled() -> bool {
+    env_bool("NYASH_JOINIR_MAINLINE_DEBUG")
+}
+
+/// NYASH_JOINIR_IF_MERGE=1 - Enable If-merge experimental mode
+pub fn if_merge_enabled() -> bool {
+    env_bool("NYASH_JOINIR_IF_MERGE")
+}
+
+/// NYASH_JOINIR_DEBUG=1 - General debug mode (deprecated, prefer mainline_debug)
+pub fn debug_enabled() -> bool {
+    env_bool("NYASH_JOINIR_DEBUG")
+}
+
+/// NYASH_JOINIR_VM_BRIDGE=1 - Enable VM bridge mode
+pub fn vm_bridge_enabled() -> bool {
+    env_bool("NYASH_JOINIR_VM_BRIDGE")
+}
+
+/// NYASH_JOINIR_STRICT=1 - Strict validation mode
+pub fn strict_enabled() -> bool {
+    env_bool("NYASH_JOINIR_STRICT")
+}
+
+/// NYASH_JOINIR_SNAPSHOT_GENERATE=1 - Generate snapshot for testing
+pub fn snapshot_generate_enabled() -> bool {
+    env_bool("NYASH_JOINIR_SNAPSHOT_GENERATE")
+}
+
+/// NYASH_JOINIR_SNAPSHOT_TEST=1 - Test using snapshot
+pub fn snapshot_test_enabled() -> bool {
+    env_bool("NYASH_JOINIR_SNAPSHOT_TEST")
+}
+
+/// NYASH_JOINIR_INPUT=* - Input source or mode
+pub fn input_mode() -> Option<String> {
+    std::env::var("NYASH_JOINIR_INPUT").ok()
+}