feat(stageb): implement UsingResolverBox foundation (partial)

Implemented:
- UsingResolverBox full implementation in using_resolver_box.hako
  - state_new(): Empty state creation
  - load_modules_json(): Load modules JSON from nyash.toml
  - resolve_path_alias(): Resolve paths from aliases
  - resolve_namespace_alias(): Tail segment matching with case-insensitive support
  - to_context_json(): Generate context JSON for ParserBox
- Added sh_core entry to nyash.toml modules section
  - Maps to lang/src/shared/common/string_helpers.hako
  - Fixes "using not found: 'sh_core'" errors
- Cleaned up compiler_stageb.hako
  - Removed problematic using statements
  - Added documentation

Known Issue (to be fixed next):
- Body extraction bug in compiler_stageb.hako:51-197
  - Multiline source extraction fails for "static box Main { main() {...} }"
  - Results in empty Program JSON body
  - Causes Stage-B emit pipeline to fall back to jsonfrag (ratio=207900%)
  - This is the root cause blocking selfhost builder path

Impact:
- ✅ sh_core resolution errors fixed
- ✅ UsingResolverBox infrastructure complete
- ❌ Stage-B emit pipeline not restored (body extraction bug)
- ❌ Selfhost builder path still blocked

Next Priority: Fix body extraction bug to restore Stage-B pipeline

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

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

lang/src/llvm_ir/boxes/aot_prep/helpers/common.hako

@@ -9,6 +9,31 @@ static box AotPrepHelpers {
     return me._linear_expr(json, vid, 0)
   }
 
+  // Public: fold integer binops on constants (returns string i64 or "")
+  evaluate_binop_constant(operation, lhs_val, rhs_val) {
+    if operation == "" { return "" }
+    local li = StringHelpers.to_i64(lhs_val)
+    local ri = StringHelpers.to_i64(rhs_val)
+    if li == null || ri == null { return "" }
+    local res = null
+    if operation == "add" || operation == "+" {
+      res = li + ri
+    } else if operation == "sub" || operation == "-" {
+      res = li - ri
+    } else if operation == "mul" || operation == "*" {
+      res = li * ri
+    } else if operation == "sdiv" || operation == "div" || operation == "/" {
+      if ri == 0 { return "" }
+      res = li / ri
+    } else if operation == "srem" || operation == "rem" || operation == "%" {
+      if ri == 0 { return "" }
+      res = li % ri
+    } else {
+      return ""
+    }
+    return StringHelpers.int_to_str(res)
+  }
+
   // Public: Is `vid` defined by a const (following copy chains)
   is_const_vid(json, vid) {
     if vid == "" { return false }
@@ -111,4 +136,3 @@ static box AotPrepHelpers {
     return -1
   }
 }
-

lang/src/llvm_ir/boxes/aot_prep/passes/loop_hoist.hako

@@ -1,7 +1,7 @@
 // AotPrepLoopHoistBox — Hoist loop-local consts (binop/div/rem/compare) to block head
 using selfhost.shared.json.utils.json_frag as JsonFragBox
 using selfhost.shared.common.string_helpers as StringHelpers
-using selfhost.llvm.ir.aot_prep as AotPrepBox  // for _evaluate_binop_constant
+using selfhost.llvm.ir.aot_prep.helpers.common as AotPrepHelpers  // for evaluate_binop_constant
 
 static box AotPrepLoopHoistBox {
   run(json) {
@@ -75,7 +75,7 @@ static box AotPrepLoopHoistBox {
           local lhs_val = const_vals.contains(lhs) ? const_vals[lhs] : ""
           local rhs_val = const_vals.contains(rhs) ? const_vals[rhs] : ""
           if lhs_val == "" || rhs_val == "" { continue }
-          local computed = AotPrepBox._evaluate_binop_constant(operation, lhs_val, rhs_val)
+          local computed = AotPrepHelpers.evaluate_binop_constant(operation, lhs_val, rhs_val)
           if computed == "" { continue }
           const_defs[dst] = inst
           const_vals[dst] = computed