feat(#1707): no-path-literal-in-assert AST rule + portability foundation (#1710)

Phase 1 of epic #1702. Closes #1707.

eslint.config.mjs

@@ -15,6 +15,7 @@ import noElapsedAssertion from './eslint-rules/no-elapsed-assertion.cjs';
 import noRawRmsyncInTests from './eslint-rules/no-raw-rmsync-in-tests.cjs';
 import noTautologicalAssert from './eslint-rules/no-tautological-assert.cjs';
 import noAdhocMarkdownParsing from './eslint-rules/no-adhoc-markdown-parsing.cjs';
+import noPathLiteralInAssert from './eslint-rules/no-path-literal-in-assert.cjs';
 
 const localPlugin = {
   rules: {
@@ -24,6 +25,7 @@ const localPlugin = {
     'no-raw-rmsync-in-tests': noRawRmsyncInTests,
     'no-tautological-assert': noTautologicalAssert,
     'no-adhoc-markdown-parsing': noAdhocMarkdownParsing,
+    'no-path-literal-in-assert': noPathLiteralInAssert,
   },
 };
 
@@ -265,6 +267,8 @@ export default tseslint.config(
       'local/no-tautological-assert': 'error',
       // Ban source-grep pattern in tests — use require() + behavior assertions instead
       'local/no-source-grep': 'error',
+      // Ban path-returning calls compared to hardcoded POSIX-slash literals (fails on Windows)
+      'local/no-path-literal-in-assert': 'error',
       // Ban raw setTimeout sync + elapsed/duration-style assertions via no-restricted-syntax
       'no-restricted-syntax': [
         'error',