feat(phase21.5/22.1): MirBuilder JsonFrag refactor + FileBox ring-1 + registry tests

Phase 21.5 (AOT/LLVM Optimization Prep)
- FileBox ring-1 (core-ro) provider: priority=-100, always available, no panic path
  - src/runner/modes/common_util/provider_registry.rs: CoreRoFileProviderFactory
  - Auto-registers at startup, eliminates fallback panic structurally
- StringBox fast path prototypes (length/size optimization)
- Performance benchmarks (C/Python/Hako comparison baseline)

Phase 22.1 (JsonFrag Unification)
- JsonFrag.last_index_of_from() for backward search (VM fallback)
- Replace hand-written lastIndexOf in lower_loop_sum_bc_box.hako
- SentinelExtractorBox for Break/Continue pattern extraction

MirBuilder Refactor (Box → JsonFrag Migration)
- 20+ lower_*_box.hako: Box-heavy → JsonFrag text assembly
- MirBuilderMinBox: lightweight using set for dev env
- Registry-only fast path with [registry:*] tag observation
- pattern_util_box.hako: enhanced pattern matching

Dev Environment & Testing
- Dev toggles: SMOKES_DEV_PREINCLUDE=1 (point-enable), HAKO_MIR_BUILDER_SKIP_LOOPS=1
- phase2160: registry opt-in tests (array/map get/set/push/len) - content verification
- phase2034: rc-dependent → token grep (grep -F based validation)
- run_quick.sh: fast smoke testing harness
- ENV documentation: docs/ENV_VARS.md

Test Results
✅ quick phase2034: ALL GREEN (MirBuilder internal patterns)
✅ registry phase2160: ALL GREEN (array/map get/set/push/len)
✅ rc-dependent tests → content token verification complete
✅ PREINCLUDE policy: default OFF, point-enable only where needed

Technical Notes
- No INCLUDE by default (maintain minimalism)
- FAIL_FAST=0 in Bring-up contexts only (explicit dev toggles)
- Tag-based route observation ([mirbuilder/min:*], [registry:*])
- MIR structure validation (not just rc parity)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

tools/ny_mir_builder.sh

@@ -24,15 +24,18 @@ TARGET=""
 NYRT_DIR=""
 VERIFY=0
 QUIET=0
-# 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.
+# Backend selection (21.13): default to 'crate' when ny-llvmc is available,
+# otherwise use 'native' when llc exists. llvmlite is deprecated from auto-select
+# and must be requested explicitly via NYASH_LLVM_BACKEND=llvmlite.
 if [[ -n "${NYASH_LLVM_BACKEND:-}" ]]; then
   BACKEND="${NYASH_LLVM_BACKEND}"
 else
   if [[ -x "./target/release/ny-llvmc" ]]; then
     BACKEND="crate"
+  elif command -v llc >/dev/null 2>&1; then
+    BACKEND="native"
   else
-    BACKEND="llvmlite"
+    BACKEND="crate"  # keep for downstream case handling; will error gracefully later
   fi
 fi
 
@@ -61,7 +64,7 @@ if [[ -z "$OUT" ]]; then
   esac
 fi
 
-# Require LLVM18 only for llvmlite backend
+# Require LLVM18 only for llvmlite backend (deprecated from auto-select)
 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
@@ -144,6 +147,17 @@ case "$EMIT" in
           echo "error: llc not found (install LLVM tools)" >&2; exit 4
         fi
         rm -f "$OUT"
+        # Optional verify: dump IR first and reject if PHI appears (simple guard)
+        if [[ "${NYASH_LLVM_VERIFY_IR:-0}" == "1" ]]; then
+          _TMP_LL=$(mktemp)
+          if ! python3 "$PWD/tools/native_llvm_builder.py" --in "$IN_FILE" --emit ll --out "$_TMP_LL" >/dev/null 2>&1; then
+            echo "error: native builder failed (ll)" >&2; rm -f "$_TMP_LL"; exit 4
+          fi
+          if grep -qE "[^a-zA-Z]phi[^a-zA-Z]" "$_TMP_LL"; then
+            echo "error: IR verify failed (phi present)" >&2; rm -f "$_TMP_LL"; exit 4
+          fi
+          rm -f "$_TMP_LL"
+        fi
         if ! python3 "$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
@@ -179,6 +193,16 @@ case "$EMIT" in
         if ! command -v llc >/dev/null 2>&1; then
           echo "error: llc not found (install LLVM tools)" >&2; exit 4
         fi
+        if [[ "${NYASH_LLVM_VERIFY_IR:-0}" == "1" ]]; then
+          _TMP_LL=$(mktemp)
+          if ! python3 "$PWD/tools/native_llvm_builder.py" --in "$IN_FILE" --emit ll --out "$_TMP_LL" >/dev/null 2>&1; then
+            echo "error: native builder failed (ll)" >&2; rm -f "$_TMP_LL"; exit 4
+          fi
+          if grep -qE "[^a-zA-Z]phi[^a-zA-Z]" "$_TMP_LL"; then
+            echo "error: IR verify failed (phi present)" >&2; rm -f "$_TMP_LL"; exit 4
+          fi
+          rm -f "$_TMP_LL"
+        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