feat(phase22.1): JsonFrag.last_index_of_from() unified search refactor

- Add: JsonFragBox.last_index_of_from(hay, needle, pos) method
  - VM fallback: simple reverse search using substring + lastIndexOf
  - Replaces hand-written lastIndexOf calls in MIR builder

- Refactor: lower_loop_sum_bc_box.hako uses unified method
  - Line 75: Break sentinel backward search
  - Line 113: Continue sentinel backward search
  - Eliminates 2 hand-written lastIndexOf calls

- Test: json_frag_last_index_of_from_canary_vm.sh
  - Loop with break(i==3) and continue(i==2)
  - Expect: 0+1+4 = 5 (skip 2, break at 3)
  - Status: PASS ✅

Phase 22.1 ultrathink cleanup: code consolidation complete

lang/src/shared/json/utils/json_frag.hako

@@ -124,6 +124,20 @@ static box JsonFragBox {
     if idx < 0 { return -1 }
     return p2 + idx
   }
+  last_index_of_from(hay, needle, pos) {
+    // VM fallback: reverse search from pos backwards to start
+    if hay == null || needle == null { return -1 }
+    local s = "" + hay
+    local n = s.length()
+    local p2 = pos
+    if p2 < 0 { return -1 }
+    if p2 >= n { p2 = n - 1 }
+    // Extract substring from 0 to pos (inclusive)
+    local substr = s.substring(0, p2 + 1)
+    // Find last occurrence of needle in substring
+    local idx = substr.lastIndexOf(needle)
+    return idx
+  }
   read_digits(text, pos) { return StringHelpers.read_digits(text, pos) }
   _str_to_int(s) { return StringHelpers.to_i64(s) }
   _to_bool10(ch) { if ch == "t" { return 1 } if ch == "f" { return 0 } return null }