refactor(env): centralize NYASH_JOINIR_STRUCTURE_ONLY flag

Moved NYASH_JOINIR_STRUCTURE_ONLY environment variable handling
from inline std::env::var() in routing.rs to a centralized helper
function in src/config/env/joinir_flags.rs.

Changes:
- Added joinir_structure_only_enabled() helper function
- Replaced direct env::var() call in routing.rs with helper
- Maintains existing behavior: default ON, NYASH_JOINIR_STRUCTURE_ONLY=0/off to disable
- Follows env module centralization pattern (Box Theory: separation of concerns)

Testing:
- cargo test --lib: 1126 passed ✅
- Regression: Phase 107 VM smoke ✅
- Phase 113/114 maintained ✅

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

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

src/mir/builder/control_flow/joinir/routing.rs

@@ -183,10 +183,7 @@ impl MirBuilder {
         // Phase 196: Default to structure-first routing now that LoopBuilder is removed.
         // - Default: ON (structure_only = true) to allow JoinIR patterns to run for all loops.
         // - To revert to the previous whitelist-only behavior, set NYASH_JOINIR_STRUCTURE_ONLY=0.
-        let structure_only = match std::env::var("NYASH_JOINIR_STRUCTURE_ONLY").ok().as_deref() {
-            Some("0") | Some("off") => false,
-            _ => true,
-        };
+        let structure_only = crate::config::env::joinir_structure_only_enabled();
 
         if structure_only {
             trace::trace().routing(