Phase 22.x WIP: LLVM backend improvements + MIR builder enhancements

LLVM backend improvements:
- Add native LLVM backend support (NYASH_LLVM_BACKEND=native)
- Add crate backend selector with priority (crate > llvmlite)
- Add native_llvm_builder.py for native IR generation
- Add NYASH_LLVM_NATIVE_TRACE=1 for IR dump

MIR builder enhancements:
- Refactor lower_if_compare_* boxes for better code generation
- Refactor lower_return_* boxes for optimized returns
- Refactor lower_loop_* boxes for loop handling
- Refactor lower_method_* boxes for method calls
- Update pattern_util_box for better pattern matching

Smoke tests:
- Add phase2100 S3 backend selector tests (17 new tests)
- Add phase2120 native backend tests (4 new tests)
- Add phase2034 MIR builder internal tests (2 new tests)
- Add phase2211 TLV shim parity test

Documentation:
- Update ENV_VARS.md with LLVM backend variables
- Update CURRENT_TASK.md with progress
- Update README.md and CHANGELOG.md

Config:
- Add NYASH_LLVM_BACKEND env support in src/config/env.rs
- Update ny_mir_builder.sh for backend selection
- Update dispatch.rs for backend routing

Tools:
- Add tools/native_llvm_builder.py
- Update smokes/v2/profiles/quick/core/phase2100/run_all.sh

Known: Many Hako builder internal files modified for optimization

src/runner/dispatch.rs

@@ -12,30 +12,9 @@ pub(crate) fn execute_file_with_backend(runner: &NyashRunner, filename: &str) {
     // Note: hv1 direct route is now handled at main.rs entry point (before NyashRunner creation).
     // This function is only called after plugins and runner initialization have already occurred.
 
-    // Selfhost pipeline (Ny -> JSON v0)
-    // Default: ON. Backward‑compat envs:
-    //  - NYASH_USE_NY_COMPILER={1|true|on} to force ON
-    //  - NYASH_USE_NY_COMPILER={0|false|off} or NYASH_DISABLE_NY_COMPILER/HAKO_DISABLE_NY_COMPILER to disable
-    let mut use_selfhost = true;
-    if let Ok(v) = std::env::var("NYASH_USE_NY_COMPILER") {
-        let lv = v.trim().to_ascii_lowercase();
-        use_selfhost = matches!(lv.as_str(), "1" | "true" | "on");
-    }
-    let disabled = std::env::var("NYASH_DISABLE_NY_COMPILER")
-        .ok()
-        .map(|v| {
-            let lv = v.trim().to_ascii_lowercase();
-            matches!(lv.as_str(), "1" | "true" | "on")
-        })
-        .unwrap_or(false)
-        || std::env::var("HAKO_DISABLE_NY_COMPILER")
-            .ok()
-            .map(|v| {
-                let lv = v.trim().to_ascii_lowercase();
-                matches!(lv.as_str(), "1" | "true" | "on")
-            })
-            .unwrap_or(false);
-    if use_selfhost && !disabled {
+    // Selfhost pipeline (Ny -> JSON v0) — consolidated env toggle
+    // Primary: NYASH_USE_NY_COMPILER=0|1; legacy disables accepted with deprecation warning
+    if crate::config::env::use_ny_compiler() {
         if runner.try_run_selfhost_pipeline(filename) {
             return;
         } else {