docs: explain path alias private import fixture

fixtures/invalid/path-alias-private-import/README.md

@@ -1,3 +1,43 @@
 # path-alias-private-import
 
-Fixture for tsconfig paths alias resolving to a private cross-cell import.
+This fixture shows that a TypeScript path alias does not bypass a cell's public boundary.
+
+## Files and cells
+
+[`cellfence.manifest.json`](./cellfence.manifest.json) defines two cells:
+
+- `producer` owns `src/producer/**` and exposes only [`src/producer/public.ts`](./src/producer/public.ts), whose allowed public symbol is `producerValue`.
+- `consumer` owns `src/consumer/**`, exposes [`src/consumer/public.ts`](./src/consumer/public.ts), and declares that it consumes `producer`.
+
+The consume relationship permits the consumer to use the producer's public entry. It does not make every file under `src/producer/` public.
+
+## Why the fixture fails
+
+[`tsconfig.json`](./tsconfig.json) maps `@app/*` to `src/*`. The import in [`src/consumer/public.ts`](./src/consumer/public.ts) therefore resolves as follows:
+
+```text
+@app/producer/private
+→ src/producer/private.ts
+```
+
+That target is [`src/producer/private.ts`](./src/producer/private.ts), not the producer cell's declared public entry. CellFence resolves the alias before checking the ownership boundary, so the cross-cell private import produces `CELLFENCE_PRIVATE_IMPORT`.
+
+The fixture also expects `CELLFENCE_OWNERSHIP_COVERAGE_DISABLED` as a warning because ownership-coverage enforcement is not enabled for this fixture. The complete expected result is recorded in [`expected-result.json`](./expected-result.json).
+
+## Reproduce
+
+From the repository root:
+
+```bash
+npm ci
+npm run build
+
+node packages/cli/dist/index.js check \
+  --root fixtures/invalid/path-alias-private-import \
+  --format markdown
+
+node --test tests/fixtures.test.mjs
+git diff --check
+```
+
+The direct `check` command is expected to exit unsuccessfully because this is an intentionally invalid fixture. Its report should contain the `CELLFENCE_PRIVATE_IMPORT` error and the `CELLFENCE_OWNERSHIP_COVERAGE_DISABLED` warning. The fixture test suite should pass by matching those findings against `expected-result.json`.