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>

src/llvm_py/instructions/boxcall.py

@@ -118,7 +118,26 @@ def lower_boxcall(
 
     # Minimal method bridging for strings and console
     if method_name in ("length", "len"):
-        # Any.length_h: Array/String/Map に対応
+        # Fast path (opt-in): pointer-based string length → nyash.string.length_si(i8*, i64 mode)
+        try:
+            import os
+            fast_on = os.environ.get('NYASH_LLVM_FAST') == '1'
+        except Exception:
+            fast_on = False
+        if fast_on and resolver is not None and hasattr(resolver, 'string_ptrs'):
+            try:
+                ptr = resolver.string_ptrs.get(int(box_vid))
+            except Exception:
+                ptr = None
+            if ptr is not None:
+                mode = 1 if os.environ.get('NYASH_STR_CP') == '1' else 0
+                mode_c = ir.Constant(i64, mode)
+                callee = _declare(module, "nyash.string.length_si", i64, [i8p, i64])
+                result = builder.call(callee, [ptr, mode_c], name="strlen_si")
+                if dst_vid is not None:
+                    vmap[dst_vid] = result
+                return
+        # Default: Any.length_h(handle) → i64
         recv_h = _ensure_handle(builder, module, recv_val)
         callee = _declare(module, "nyash.any.length_h", i64, [i64])
         result = builder.call(callee, [recv_h], name="any_length_h")