Phase 21.4 Complete: FileBox SSOT + Analyzer Stabilization (7 Tasks)

✅ Task 1: Fallback Guarantee (create_box failure → ring1/core-ro auto fallback)
- Three-tier fallback system: plugin → builtin → core-ro
- Mode control: auto/plugin-only/core-ro
- New: src/box_factory/builtin_impls/file_box.rs
- New: tools/test_filebox_fallback_smoke.sh

✅ Task 2: Provider Registration SSOT (static/dynamic/core-ro unified)
- ProviderFactory trait with priority-based selection
- Global registry PROVIDER_FACTORIES implementation
- Priority: dynamic(100) > builtin(10) > core-ro(0)
- New: src/boxes/file/builtin_factory.rs
- New: tools/smoke_provider_modes.sh

✅ Task 3: FileBox Publication Unification
- Verified: basic/file_box.rs already minimized (11 lines)
- Perfect re-export pattern maintained

✅ Task 4: ENV Unification (FILEBOX_MODE/DISABLE_PLUGINS priority)
- Removed auto-setting of NYASH_USE_PLUGIN_BUILTINS
- Removed auto-setting of NYASH_PLUGIN_OVERRIDE_TYPES
- Added deprecation warnings with migration guide
- ENV hierarchy: DISABLE_PLUGINS > BOX_FACTORY_POLICY > FILEBOX_MODE

✅ Task 5: Error Log Visibility (Analyzer rule execution errors to stderr)
- Added [rule/exec] logging before IR-based rule execution
- Format: [rule/exec] HC012 (dead_static_box) <filepath>
- VM errors now traceable via stderr output

✅ Task 6: Unnecessary Using Removal (14 rules Str alias cleanup)
- Removed unused `using ... as Str` from 14 rule files
- All rules use local _itoa() helper instead
- 14 lines of dead code eliminated

✅ Task 7: HC017 Skip & TODO Documentation (UTF-8 support required)
- Enhanced run_tests.sh with clear skip message
- Added "Known Limitations" section to README.md
- Technical requirements documented (3 implementation options)
- Re-enable timeline: Phase 22 (Unicode Support Phase)

📊 Test Results:
- Analyzer: 10 tests PASS, 1 skipped (HC017)
- FileBox fallback: All 3 modes PASS
- Provider modes: All 4 modes PASS
- Build: Success (0 errors, 0 warnings)

🎯 Key Achievements:
- 28 files modified/created
- Three-Tier Fallback System (stability)
- SSOT Provider Registry (extensibility)
- ENV unification (operational clarity)
- Error visibility (debugging efficiency)
- Code cleanup (maintainability)
- Comprehensive documentation (Phase 22 ready)

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

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

src/runner/plugins.rs

@@ -7,35 +7,42 @@ use super::*;
 impl NyashRunner {
     /// Initialize global runtime registry and load configured plugins.
     ///
-    /// Behavior (no-op changes to defaults):
+    /// Behavior (Phase 21.4: Unified ENV Policy):
     /// - Always initializes the unified type/box registry.
     /// - Loads native BID plugins unless `NYASH_DISABLE_PLUGINS=1`.
-    /// - Ensures built‑ins override list includes ArrayBox/MapBox/FileBox/TOMLBox
-    ///   by merging `NYASH_PLUGIN_OVERRIDE_TYPES` with required entries.
     /// - When `--load-ny-plugins` or `NYASH_LOAD_NY_PLUGINS=1` is set, best‑effort
     ///   loads Nyash scripts listed under `nyash.toml`'s `ny_plugins`.
     ///
-    /// Side effects:
-    /// - Sets `NYASH_USE_PLUGIN_BUILTINS=1` if unset, to allow interpreter side creation.
-    /// - Updates `NYASH_PLUGIN_OVERRIDE_TYPES` env var (merged values).
+    /// ENV Policy:
+    /// - NYASH_DISABLE_PLUGINS=1: Skip all plugin initialization
+    /// - NYASH_BOX_FACTORY_POLICY: Control factory priority (builtin_first|strict_plugin_first|compat_plugin_first)
+    /// - NYASH_FILEBOX_MODE: FileBox provider selection (auto|core-ro|plugin-only)
+    ///
+    /// Deprecated ENV (removed auto-setting):
+    /// - NYASH_USE_PLUGIN_BUILTINS: No longer auto-set (use policy instead)
+    /// - NYASH_PLUGIN_OVERRIDE_TYPES: No longer auto-set (use policy instead)
     pub(crate) fn init_runtime_and_plugins(&self, groups: &crate::cli::CliGroups) {
-        // Unified registry
+        // Check if plugins are disabled
+        let plugins_disabled = std::env::var("NYASH_DISABLE_PLUGINS").ok().as_deref() == Some("1");
+
+        // Unified registry (always initialize)
         runtime::init_global_unified_registry();
-        // Plugins (guarded)
-        if std::env::var("NYASH_DISABLE_PLUGINS").ok().as_deref() != Some("1") {
+
+        // Plugins (guarded by NYASH_DISABLE_PLUGINS)
+        if !plugins_disabled {
             runner_plugin_init::init_bid_plugins();
             crate::runner::box_index::refresh_box_index();
+        } else {
+            eprintln!("[plugins] Skipping plugin initialization (NYASH_DISABLE_PLUGINS=1)");
         }
-        // Allow interpreter to create plugin-backed boxes via unified registry
-        if std::env::var("NYASH_USE_PLUGIN_BUILTINS").ok().is_none() {
-            std::env::set_var("NYASH_USE_PLUGIN_BUILTINS", "1");
+
+        // Deprecation warnings for old ENV variables
+        if std::env::var("NYASH_USE_PLUGIN_BUILTINS").is_ok() {
+            eprintln!("[warn] NYASH_USE_PLUGIN_BUILTINS is deprecated. Use NYASH_BOX_FACTORY_POLICY instead.");
+        }
+        if std::env::var("NYASH_PLUGIN_OVERRIDE_TYPES").is_ok() {
+            eprintln!("[warn] NYASH_PLUGIN_OVERRIDE_TYPES is deprecated. Use NYASH_BOX_FACTORY_POLICY instead.");
         }
-        // Merge override types env (ensure FileBox/TOMLBox present)
-        let mut override_types: Vec<String> = if let Ok(list) = std::env::var("NYASH_PLUGIN_OVERRIDE_TYPES") {
-            list.split(',').map(|s| s.trim().to_string()).filter(|s| !s.is_empty()).collect()
-        } else { vec!["ArrayBox".into(), "MapBox".into()] };
-        for t in ["FileBox", "TOMLBox"] { if !override_types.iter().any(|x| x == t) { override_types.push(t.into()); } }
-        std::env::set_var("NYASH_PLUGIN_OVERRIDE_TYPES", override_types.join(","));
 
         // Optional Ny script plugins loader (best-effort)
         if groups.load_ny_plugins || std::env::var("NYASH_LOAD_NY_PLUGINS").ok().as_deref() == Some("1") {