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

README.md

@@ -4,6 +4,18 @@
 **A Seriously-Crafted Hobby Language**  
 **From Zero to Native Binary in 20 Days - The AI-Powered Language Revolution**
 
+Quick — Emit MIR (Hako‑first helper)
+- Generate MIR(JSON) from a Hako file using the Stage‑B parser + MirBuilder (wrapper falls back to the Rust CLI builder on failure to keep runs green):
+
+```
+tools/hakorune_emit_mir.sh path/to/program.hako /tmp/program.mir.json
+target/release/hakorune --mir-json-file /tmp/program.mir.json
+```
+
+Notes
+- The wrapper runs Stage‑B with `NYASH_JSON_ONLY=1` to keep the output clean (no `RC:` lines).
+- When the Hako MirBuilder fails (e.g., under development), it automatically falls back to `--program-json-to-mir` (no behavior change by default).
+
 *[🇯🇵 日本語版はこちら / Japanese Version](README.ja.md)*
 
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