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

crates/nyash_kernel_min_c/src/kernel_min.c

@@ -17,8 +17,6 @@ long nyash_box_from_i8_string(char* p) {
     return 0;
 }
 
-// Optional minimal stubs (not used by default; reserved for future reps)
-long nyash_array_birth_h(void) { return 1; }
-long nyash_array_length_h(long handle) { (void)handle; return 0; }
-long nyash_map_birth_h(void) { return 1; }
-long nyash_map_size_h(long handle) { (void)handle; return 0; }
+// Note: Additional array/map stubs intentionally omitted to avoid symbol
+// clashes with the full NyKernel when linked together. Keep this file minimal
+// (console only) for print canaries.