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/main.rs

@@ -13,8 +13,7 @@ fn main() {
     // If NYASH_VERIFY_JSON is present and route is requested, execute and exit.
     // This avoids plugin host/registry initialization and keeps output minimal.
     let has_json = std::env::var("NYASH_VERIFY_JSON").is_ok();
-    let route = nyash_rust::config::env::env_bool("HAKO_ROUTE_HAKOVM")
-        || std::env::var("HAKO_VERIFY_PRIMARY").ok().as_deref() == Some("hakovm");
+    let route = nyash_rust::config::env::verify_primary_is_hakovm();
     // Force flag may allow hv1-inline without route
     let force_hv1_flag = nyash_rust::config::env::env_bool("HAKO_VERIFY_V1_FORCE_HAKOVM");
     if has_json && (route || force_hv1_flag) {