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>

crates/nyash_kernel/src/plugin/string.rs

@@ -140,6 +140,35 @@ pub extern "C" fn nyash_string_lastindexof_ss(s: *const i8, needle: *const i8) -
     }
 }
 
+// Exported as: nyash.string.length_si(i8* s, i64 mode) -> i64
+// mode: 0 = byte length (UTF-8 bytes), 1 = char length (Unicode scalar count)
+#[export_name = "nyash.string.length_si"]
+pub extern "C" fn nyash_string_length_si(s: *const i8, mode: i64) -> i64 {
+    use std::ffi::CStr;
+    if s.is_null() {
+        return 0;
+    }
+    let cs = unsafe { CStr::from_ptr(s) };
+    // Safe UTF-8 conversion; on failure, fall back to byte length scan
+    if let Ok(st) = cs.to_str() {
+        if mode == 1 { // char count
+            return st.chars().count() as i64;
+        } else { // byte length
+            return st.as_bytes().len() as i64;
+        }
+    }
+    // Fallback: manual byte scan to NUL
+    let mut len: i64 = 0;
+    unsafe {
+        let mut p = s;
+        while *p != 0 {
+            len += 1;
+            p = p.add(1);
+        }
+    }
+    len
+}
+
 // Exported as: nyash.string.to_i8p_h(i64 handle) -> i8*
 #[export_name = "nyash.string.to_i8p_h"]
 pub extern "C" fn nyash_string_to_i8p_h(handle: i64) -> *mut i8 {