refactor(joinir): Phase 82-83 - Debug flag SSOT + Fallback verification

Phase 82: Centralized JoinIR debug flag reading
- Added is_joinir_debug() SSOT function in joinir_flags.rs
- Replaced 16 direct env::var() calls across 8 files
- Updated docs to recommend HAKO_JOINIR_DEBUG (NYASH_ deprecated)
- Backward compat: Both env vars work

Phase 83: Verified promoted carrier fallback behavior
- Confirmed NO fallback to name-based lookup for DigitPos/Trim
- Documented fallback expectations in Phase 80/81 docs
- Added verification commands and expected output

Changes:
- src/config/env/joinir_flags.rs: +187 lines (new SSOT module)
- 8 files: env var reads → is_joinir_debug() calls
- 3 docs: HAKO_JOINIR_DEBUG examples + fallback sections
- 1 summary doc: phase82-83-debug-flag-ssot-summary.md

Tests: 970/970 lib PASS, 58/58 normalized_dev PASS
Impact: Dev-only (zero production changes)

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

@@ -64,9 +64,10 @@ pub struct JoinLoopTrace {
 impl JoinLoopTrace {
     /// Create a new tracer, reading environment variables.
     pub fn new() -> Self {
+        use crate::config::env::is_joinir_debug;
         Self {
             varmap_enabled: std::env::var("NYASH_TRACE_VARMAP").is_ok(),
-            joinir_enabled: std::env::var("NYASH_JOINIR_DEBUG").is_ok(),
+            joinir_enabled: is_joinir_debug(),
             phi_enabled: std::env::var("NYASH_OPTION_C_DEBUG").is_ok(),
             mainline_enabled: std::env::var("NYASH_JOINIR_MAINLINE_DEBUG").is_ok(),
             loopform_enabled: std::env::var("NYASH_LOOPFORM_DEBUG").is_ok(),