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

tools/ny_mir_builder.sh

@@ -24,7 +24,17 @@ TARGET=""
 NYRT_DIR=""
 VERIFY=0
 QUIET=0
-BACKEND="${NYASH_LLVM_BACKEND:-llvmlite}"  # llvmlite | crate | native (reserved)
+# Backend selection (21.11): default to 'crate' when ny-llvmc is available, otherwise fallback to llvmlite.
+# Explicit env NYASH_LLVM_BACKEND overrides this auto-detection.
+if [[ -n "${NYASH_LLVM_BACKEND:-}" ]]; then
+  BACKEND="${NYASH_LLVM_BACKEND}"
+else
+  if [[ -x "./target/release/ny-llvmc" ]]; then
+    BACKEND="crate"
+  else
+    BACKEND="llvmlite"
+  fi
+fi
 
 while [[ $# -gt 0 ]]; do
   case "$1" in
@@ -51,9 +61,12 @@ if [[ -z "$OUT" ]]; then
   esac
 fi
 
-if ! command -v llvm-config-18 >/dev/null 2>&1; then
-  echo "error: llvm-config-18 not found (install LLVM 18 dev)" >&2
-  exit 3
+# Require LLVM18 only for llvmlite backend
+if [[ "${NYASH_LLVM_BACKEND:-$BACKEND}" == "llvmlite" ]]; then
+  if ! command -v llvm-config-18 >/dev/null 2>&1; then
+    echo "error: llvm-config-18 not found (install LLVM 18 dev)" >&2
+    exit 3
+  fi
 fi
 
 # Build nyash + NyRT as needed（skip allowed）
@@ -126,6 +139,15 @@ case "$EMIT" in
         rm -f "$OUT"
         "$BIN_NYLLVMC" --in "$IN_FILE" --emit obj --out "$OUT" >/dev/null 2>&1 || { echo "error: ny-llvmc failed" >&2; exit 4; }
         ;;
+      native)
+        if ! command -v llc >/dev/null 2>&1; then
+          echo "error: llc not found (install LLVM tools)" >&2; exit 4
+        fi
+        rm -f "$OUT"
+        if ! python3 "$PWD/tools/native_llvm_builder.py" --in "$IN_FILE" --emit obj --out "$OUT" >/dev/null 2>&1; then
+          echo "error: native builder failed" >&2; exit 4
+        fi
+        ;;
       llvmlite|*)
         # Directly use llvmlite harness with MIR v1 JSON input
         rm -f "$OUT"
@@ -153,6 +175,22 @@ case "$EMIT" in
           echo "error: ny-llvmc failed to link exe" >&2; exit 4
         fi
         ;;
+      native)
+        if ! command -v llc >/dev/null 2>&1; then
+          echo "error: llc not found (install LLVM tools)" >&2; exit 4
+        fi
+        if ! python3 "$PWD/tools/native_llvm_builder.py" --in "$IN_FILE" --emit obj --out "$OBJ" >/dev/null 2>&1; then
+          echo "error: native builder failed to produce object $OBJ" >&2; exit 4
+        fi
+        if [[ ! -f "$OBJ" ]]; then echo "error: failed to produce object $OBJ" >&2; exit 4; fi
+        # Link with NyRT (same as llvmlite branch)
+        NYRT_BASE=${NYRT_DIR:-"$PWD/crates/nyash_kernel"}
+        cc "$OBJ" \
+          -L target/release \
+          -L "$NYRT_BASE/target/release" \
+          -Wl,--whole-archive -lnyash_kernel -Wl,--no-whole-archive \
+          -lpthread -ldl -lm -o "$OUT"
+        ;;
       llvmlite|*)
         if ! python3 "$PWD/tools/llvmlite_harness.py" --in "$IN_FILE" --out "$OBJ" >/dev/null 2>&1; then
           echo "error: harness failed to produce object $OBJ" >&2; exit 4