fix: C backend String.indexOf/lastIndexOf compilation (issue 8)

Three bugs fixed:
- Global let bindings always typed as LuxInt; now inferred from value
- Option inner type not tracked for function params; added
  var_option_inner_types map so match extraction uses correct type
- indexOf/lastIndexOf stored ints as (void*)(intptr_t) but extraction
  expected boxed pointers; now uses lux_box_int consistently

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Cargo.lock

@@ -770,7 +770,7 @@ dependencies = [
 
 [[package]]
 name = "lux"
-version = "0.1.5"
+version = "0.1.6"
 dependencies = [
  "lsp-server",
  "lsp-types",