feat(phase21.5): MirBuilder optimization prep + crate EXE infrastructure

Phase 21.5 optimization readiness - C-level performance target:
- MirBuilder: JsonFrag purify toggle (HAKO_MIR_BUILDER_JSONFRAG_PURIFY=1)
- Normalizer: extended f64 canonicalization + dedupe improvements
- loop_opts_adapter: JsonFrag path refinement for crate EXE compatibility

Infrastructure improvements:
- provider_registry: add diagnostics + ring-1 providers (array/console/map/path)
- mir_interpreter: add normalization/purify feature gates
- tools/selfhost_exe_stageb.sh: new end-to-end Stage-B→crate EXE pipeline
- tools/perf/microbench.sh: performance measurement tooling

Smoke tests (phase2100):
- Extend timeout 15s→120s for heavy crate EXE builds
- Add stageb_loop_jsonfrag_crate_exe_canary_vm.sh (target test)
- Add s3_backend_selector_crate_exe_vm_parity_return42_canary_vm.sh

Documentation:
- ENV_VARS.md: add Phase 21.5 optimization toggles
- README updates: clarify crate backend strategy
- phase215-optimization.md: new optimization roadmap

This commit sets the stage for Phase 21.5 critical optimization:
achieving C-level performance to decide hakorune's future viability.

src/config/mod.rs

@@ -4,5 +4,6 @@
 
 pub mod env;
 pub mod nyash_toml_v2;
+pub mod provider_env;
 
 pub use nyash_toml_v2::{BoxTypeConfig, LibraryDefinition, MethodDefinition, NyashConfigV2};

src/config/provider_env.rs

@@ -0,0 +1,47 @@
+//! Provider-related environment readers (centralized)
+//!
+//! Minimizes scattered `std::env::var` calls across the runtime by
+//! providing a small typed surface for provider selection knobs.
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum ProviderPolicy {
+    /// Current default: prefer plugin/dynamic providers by priority; use ring-1 as fallback
+    StrictPluginFirst,
+    /// Prefer ring-1 (static/core-ro) for stability/CI; fall back to plugin if unavailable
+    SafeCoreFirst,
+    /// Alias to SafeCoreFirst for future extension
+    StaticPreferred,
+}
+
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+pub enum FileBoxMode { Auto, CoreRo, PluginOnly }
+
+/// Read global provider policy (affects Auto mode only)
+pub fn provider_policy_from_env() -> ProviderPolicy {
+    match std::env::var("HAKO_PROVIDER_POLICY").unwrap_or_else(|_| "strict-plugin-first".to_string()).as_str() {
+        "safe-core-first" => ProviderPolicy::SafeCoreFirst,
+        "static-preferred" => ProviderPolicy::StaticPreferred,
+        _ => ProviderPolicy::StrictPluginFirst,
+    }
+}
+
+/// Read FileBox mode from environment variables
+pub fn filebox_mode_from_env() -> FileBoxMode {
+    match std::env::var("NYASH_FILEBOX_MODE").unwrap_or_else(|_| "auto".to_string()).as_str() {
+        "core-ro" => FileBoxMode::CoreRo,
+        "plugin-only" => FileBoxMode::PluginOnly,
+        _ => {
+            if std::env::var("NYASH_DISABLE_PLUGINS").as_deref() == Ok("1") {
+                FileBoxMode::CoreRo
+            } else { FileBoxMode::Auto }
+        }
+    }
+}
+
+/// Allow core-ro fallback override even if Fail-Fast is ON when:
+/// - JSON-only pipeline is active (quiet structured I/O), or
+/// - Explicit NYASH_FILEBOX_ALLOW_FALLBACK=1 is set.
+pub fn allow_filebox_fallback_override(quiet_pipe: bool) -> bool {
+    quiet_pipe || crate::config::env::env_bool("NYASH_FILEBOX_ALLOW_FALLBACK")
+}
+