feat(phase21.5/22.1): MirBuilder JsonFrag refactor + FileBox ring-1 + registry tests

Phase 21.5 (AOT/LLVM Optimization Prep)
- FileBox ring-1 (core-ro) provider: priority=-100, always available, no panic path
  - src/runner/modes/common_util/provider_registry.rs: CoreRoFileProviderFactory
  - Auto-registers at startup, eliminates fallback panic structurally
- StringBox fast path prototypes (length/size optimization)
- Performance benchmarks (C/Python/Hako comparison baseline)

Phase 22.1 (JsonFrag Unification)
- JsonFrag.last_index_of_from() for backward search (VM fallback)
- Replace hand-written lastIndexOf in lower_loop_sum_bc_box.hako
- SentinelExtractorBox for Break/Continue pattern extraction

MirBuilder Refactor (Box → JsonFrag Migration)
- 20+ lower_*_box.hako: Box-heavy → JsonFrag text assembly
- MirBuilderMinBox: lightweight using set for dev env
- Registry-only fast path with [registry:*] tag observation
- pattern_util_box.hako: enhanced pattern matching

Dev Environment & Testing
- Dev toggles: SMOKES_DEV_PREINCLUDE=1 (point-enable), HAKO_MIR_BUILDER_SKIP_LOOPS=1
- phase2160: registry opt-in tests (array/map get/set/push/len) - content verification
- phase2034: rc-dependent → token grep (grep -F based validation)
- run_quick.sh: fast smoke testing harness
- ENV documentation: docs/ENV_VARS.md

Test Results
✅ quick phase2034: ALL GREEN (MirBuilder internal patterns)
✅ registry phase2160: ALL GREEN (array/map get/set/push/len)
✅ rc-dependent tests → content token verification complete
✅ PREINCLUDE policy: default OFF, point-enable only where needed

Technical Notes
- No INCLUDE by default (maintain minimalism)
- FAIL_FAST=0 in Bring-up contexts only (explicit dev toggles)
- Tag-based route observation ([mirbuilder/min:*], [registry:*])
- MIR structure validation (not just rc parity)

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

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

src/runner/modes/common_util/provider_registry.rs

@@ -28,6 +28,27 @@ pub fn register_provider_factory(factory: Arc<dyn ProviderFactory>) {
     registry.lock().unwrap().push(factory);
 }
 
+/// Built‑in ring‑1 FileBox provider (core‑ro) — always available, lowest priority
+struct CoreRoFileProviderFactory;
+
+impl ProviderFactory for CoreRoFileProviderFactory {
+    fn box_name(&self) -> &str { "FileBox" }
+    fn create_provider(&self) -> Arc<dyn FileIo> { Arc::new(CoreRoFileIo::new()) }
+    fn is_available(&self) -> bool { true }
+    fn priority(&self) -> i32 { -100 } // ring‑1: lower than any plugin/provider
+}
+
+/// Ensure ring‑1 (core‑ro) provider is present in the registry
+fn ensure_builtin_file_provider_registered() {
+    let reg = PROVIDER_FACTORIES.get_or_init(|| Mutex::new(Vec::new()));
+    let mut guard = reg.lock().unwrap();
+    // If at least one FileBox provider exists, we still keep ring‑1 present for safety; avoid duplicates by checking any core‑ro present by priority
+    let has_core_ro = guard.iter().any(|f| f.box_name() == "FileBox" && f.priority() <= -100);
+    if !has_core_ro {
+        guard.push(Arc::new(CoreRoFileProviderFactory));
+    }
+}
+
 /// Read FileBox mode from environment variables
 #[allow(dead_code)]
 pub fn read_filebox_mode_from_env() -> FileBoxMode {
@@ -45,8 +66,10 @@ pub fn read_filebox_mode_from_env() -> FileBoxMode {
 /// Select provider based on mode and registered factories (SSOT)
 #[allow(dead_code)]
 pub fn select_file_provider(mode: FileBoxMode) -> Arc<dyn FileIo> {
-    let registry = PROVIDER_FACTORIES.get();
     let quiet_pipe = crate::config::env::env_bool("NYASH_JSON_ONLY");
+    // Always ensure ring‑1 (core‑ro) exists before inspecting registry
+    ensure_builtin_file_provider_registered();
+    let registry = PROVIDER_FACTORIES.get();
 
     match mode {
         FileBoxMode::Auto => {
@@ -69,11 +92,27 @@ pub fn select_file_provider(mode: FileBoxMode) -> Arc<dyn FileIo> {
                 }
             }
 
-            // Fallback to core-ro
-            if !quiet_pipe {
-                eprintln!("[provider-registry] FileBox: using core-ro fallback");
+            // Fallback policy
+            // Allow a narrow, explicit carve‑out:
+            // - When JSON‑only pipeline is active (quiet structured I/O), or
+            // - When NYASH_FILEBOX_ALLOW_FALLBACK=1 is set,
+            // always use core‑ro provider even if Fail‑Fast is ON.
+            let allow_fb_override =
+                crate::config::env::env_bool("NYASH_JSON_ONLY") ||
+                crate::config::env::env_bool("NYASH_FILEBOX_ALLOW_FALLBACK");
+
+            if crate::config::env::fail_fast() && !allow_fb_override {
+                eprintln!("[failfast/provider/filebox:auto-fallback-blocked]");
+                panic!("Fail-Fast: FileBox provider fallback is disabled (NYASH_FAIL_FAST=0 or NYASH_FILEBOX_ALLOW_FALLBACK=1 to override)");
+            } else {
+                if !quiet_pipe {
+                    eprintln!(
+                        "[provider-registry] FileBox: using core-ro fallback{}",
+                        if allow_fb_override { " (override)" } else { "" }
+                    );
+                }
+                Arc::new(CoreRoFileIo::new())
             }
-            Arc::new(CoreRoFileIo::new())
         }
         FileBoxMode::PluginOnly => {
             // Try only registered providers, Fail-Fast if none available
@@ -105,4 +144,3 @@ pub fn select_file_provider(mode: FileBoxMode) -> Arc<dyn FileIo> {
         }
     }
 }
-