Add project-structure module: the same Boot 4.1 app laid out package-by-layer and package-by-feature, with the public surface counted and the javac boundary captured

Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01HhW8LD8r3EigoBaCGrQZx5 Co-Authored-By: Claude Sonnet 5.5 <[email protected]> Claude-Session: https://claude.ai/code/session_01HhW8LD8r3EigoBaCGrQZx5
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+# project-structure
+
+Companion project for the article **[Spring Boot Project Structure: Package-by-Layer vs Package-by-Feature](https://ankurm.com/spring-boot-project-structure/)** on **[ankurm.com](https://ankurm.com)**.
+
+One small library application (books, plus loans in the feature layout) written twice, with the same
+HTTP API, so the only difference between the two is where the files live. Every console block quoted in
+the article came out of `output/`, written by the test suite, so a claim that stops being true turns the
+build red.
+
+There is deliberately **no `docs/` folder**: the deeper material lives in collapsible sections inside the
+article itself.
+
+## Versions
+
+| | |
+|---|---|
+| Spring Boot | 4.1.1 |
+| Spring Framework | 7.0.9 |
+| JDK | 21 (OpenJDK 21.0.12) |
+| Maven | 3.9 |
+
+The module was compiled and run on JDK 21 only; it has not been run on JDK 25.
+
+## Quickstart
+
+```bash
+mvn test                    # runs everything and rewrites output/01-04
+./scripts/run-all.sh        # same, then lists output/
+mvn spring-boot:run         # starts the recommended (by-feature) layout on :8080
+```
+
+## Source layout
+
+| Package | What it is |
+|---|---|
+| `bylayer/` | package-by-layer: `controller/`, `service/`, `repository/`, `model/`, `dto/`, `exception/` |
+| `byfeature/book/` | the book feature: everything package-private except `BookCatalog`, the feature's public API |
+| `byfeature/loan/` | the loan feature: uses books only through `BookCatalog` |
+| `byfeature/shared/` | cross-cutting: `ApiException` and the one `@RestControllerAdvice` |
+
+Both applications have their own `@SpringBootApplication` (`ByLayerApp`, `ByFeatureApp`), and each scans
+only its own package, so the two never see each other's beans.
+
+## Captured output
+
+| File | What it shows |
+|---|---|
+| `01-package-trees.txt` | the real source tree of both layouts (9 files vs 15 files) |
+| `02-public-surface.txt` | every top-level type and whether it is `public`: 9 of 9 by layer, 3 of 15 by feature |
+| `03-compiler-boundary.txt` | the same one-line shortcut (`new BookRepository()` from another package): compiles by layer, rejected by `javac` by feature |
+| `04-same-api-both-layouts.txt` | five identical requests against both layouts, plus the loan feature's three requests |