test: split checker_frontend into spec-tagged submodules (no logic change)

193 tests relocated from one 5079-line wall into tests/checker_frontend/{parse,
capabilities,effects,conflicts,world,resources,types,lint,misc}.rs, each headed
with the spec section it covers. Same binary, identical test set.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: Haofei

crates/rsscript/tests/checker_frontend.rs

@@ -0,0 +1,29 @@
+//! Spec §2.6/§2A — lints and review budget
+#![allow(unused_imports, dead_code)]
+use super::*;
+
+#[test]
+fn required_spec_diagnostics_have_regression_coverage() {
+    let fixture_codes = common::fail_fixture_expected_code_set();
+    let dedicated_test_codes = BTreeSet::from([
+        "RS1102",  // rustc_diagnostics_report_unmappable_generated_spans
+        "RS1201",  // runtime_diagnostic_lines_parse_to_rsscript_diagnostics
+        "RS0310",  // checker_rejects_exclusive_use_of_for_read_view
+        "PKG0101", // package feature resolution diagnostics
+        "PKG0102", // unsupported package dependency source diagnostics
+        "PKG0501", // package review policy diagnostics
+        "PKG0601", // package native binding diagnostics
+        "PKG0901", // package provider declaration diagnostics
+    ]);
+
+    for &(spec_class, code) in REQUIRED_SPEC_DIAGNOSTICS {
+        assert!(
+            explain_diagnostic_code(code).is_some(),
+            "{spec_class} maps to {code}, but the code has no explanation"
+        );
+        assert!(
+            fixture_codes.contains(code) || dedicated_test_codes.contains(code),
+            "{spec_class} maps to {code}, but no fail fixture or dedicated regression test covers it"
+        );
+    }
+}