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>

lang/src/mir/builder/internal/loop_scan_box.hako

@@ -6,13 +6,15 @@ static box LoopScanBox {
   // 抽出: cond Compare から Var 名（lhs/rhs いずれか）を取得
   find_loop_var_name(s, k_cmp) {
     local varname = null
-    local kl = ("" + s).indexOf("\"lhs\":{", k_cmp)
-    local kr = ("" + s).indexOf("\"rhs\":{", k_cmp)
-    if kl >= 0 && ("" + s).indexOf("\"type\":\"Var\"", kl) >= 0 {
-      local kn = ("" + s).indexOf("\"name\":\"", kl); if kn >= 0 { varname = JsonFragBox.read_string_after(s, kn) }
+    local kl = JsonFragBox.index_of_from(s, "\"lhs\":{", k_cmp)
+    local kr = JsonFragBox.index_of_from(s, "\"rhs\":{", k_cmp)
+    if kl >= 0 && JsonFragBox.index_of_from(s, "\"type\":\"Var\"", kl) >= 0 {
+      local kn = JsonFragBox.index_of_from(s, "\"name\":\"", kl)
+      if kn >= 0 { varname = JsonFragBox.read_string_after(s, kn) }
     }
-    if varname == null && kr >= 0 && ("" + s).indexOf("\"type\":\"Var\"", kr) >= 0 {
-      local kn2 = ("" + s).indexOf("\"name\":\"", kr); if kn2 >= 0 { varname = JsonFragBox.read_string_after(s, kn2) }
+    if varname == null && kr >= 0 && JsonFragBox.index_of_from(s, "\"type\":\"Var\"", kr) >= 0 {
+      local kn2 = JsonFragBox.index_of_from(s, "\"name\":\"", kr)
+      if kn2 >= 0 { varname = JsonFragBox.read_string_after(s, kn2) }
     }
     return varname
   }
@@ -21,14 +23,14 @@ static box LoopScanBox {
   // sentinel: "Break" | "Continue"
   extract_ne_else_sentinel_value(s, sentinel, k_loop, varname) {
     local st = "\"type\":\"" + sentinel + "\""
-    local ks = ("" + s).indexOf(st, k_loop)
+    local ks = JsonFragBox.index_of_from(s, st, k_loop)
     if ks < 0 { return null }
     // 直前の If と Compare を見つける（同一 then/else 内の近傍に限定）
-    local kif = ("" + s).lastIndexOf("\"type\":\"If\"", ks)
+    local kif = JsonFragBox.last_index_of_from(s, "\"type\":\"If\"", ks)
     if kif < 0 { return null }
-    local kcmp = ("" + s).lastIndexOf("\"type\":\"Compare\"", ks)
+    local kcmp = JsonFragBox.last_index_of_from(s, "\"type\":\"Compare\"", ks)
     if kcmp < 0 || kcmp < kif { return null }
-    local kop = ("" + s).indexOf("\"op\":", kcmp); if kop < 0 { return null }
+    local kop = JsonFragBox.index_of_from(s, "\"op\":", kcmp); if kop < 0 { return null }
     local op = JsonFragBox.read_string_after(s, kop + 5); if op == null || op != "!=" { return null }
     // else 範囲の配列区間を特定
     local kth = JsonFragBox.index_of_from(s, "\"then\":", kif); if kth < 0 { return null }
@@ -40,13 +42,13 @@ static box LoopScanBox {
     // sentinel が else ブロック中にあること
     if !(ks > lb_else && ks < rb_else) { return null }
     // 比較の反対側 Int を抽出（lhs=Var(varname) → rhs Int、rhs=Var → lhs Int）
-    local has_lhs = ("" + s).indexOf("\"lhs\":{\"type\":\"Var\",\"name\":\"" + varname + "\"}", kcmp) >= 0
-    local has_rhs = ("" + s).indexOf("\"rhs\":{\"type\":\"Var\",\"name\":\"" + varname + "\"}", kcmp) >= 0
+    local has_lhs = JsonFragBox.index_of_from(s, "\"lhs\":{\"type\":\"Var\",\"name\":\"" + varname + "\"}", kcmp) >= 0
+    local has_rhs = JsonFragBox.index_of_from(s, "\"rhs\":{\"type\":\"Var\",\"name\":\"" + varname + "\"}", kcmp) >= 0
     if !has_lhs && !has_rhs { return null }
     if has_lhs {
-      local kr = ("" + s).indexOf("\"rhs\":{", kcmp); if kr < 0 { return null }
-      local kt = ("" + s).indexOf("\"type\":\"Int\"", kr); if kt < 0 { return null }
-      local kv = ("" + s).indexOf("\"value\":", kt); if kv < 0 { return null }
+      local kr = JsonFragBox.index_of_from(s, "\"rhs\":{", kcmp); if kr < 0 { return null }
+      local kt = JsonFragBox.index_of_from(s, "\"type\":\"Int\"", kr); if kt < 0 { return null }
+      local kv = JsonFragBox.index_of_from(s, "\"value\":", kt); if kv < 0 { return null }
       local sentinel_val = JsonFragBox.read_int_after(s, kv + 8)
       // Safety check: must be valid numeric string
       if sentinel_val == null { return null }
@@ -65,9 +67,9 @@ static box LoopScanBox {
       return sentinel_val
     }
     // rhs が変数
-    local kl = ("" + s).indexOf("\"lhs\":{", kcmp); if kl < 0 { return null }
-    local kt2 = ("" + s).indexOf("\"type\":\"Int\"", kl); if kt2 < 0 { return null }
-    local kv2 = ("" + s).indexOf("\"value\":", kt2); if kv2 < 0 { return null }
+    local kl = JsonFragBox.index_of_from(s, "\"lhs\":{", kcmp); if kl < 0 { return null }
+    local kt2 = JsonFragBox.index_of_from(s, "\"type\":\"Int\"", kl); if kt2 < 0 { return null }
+    local kv2 = JsonFragBox.index_of_from(s, "\"value\":", kt2); if kv2 < 0 { return null }
     local sentinel_val2 = JsonFragBox.read_int_after(s, kv2 + 8)
     // Safety check for rhs case
     if sentinel_val2 == null { return null }