docs(contributing): name the public-API type-test rule (SD-2862) (#3042)

The auto-derive script (SD-2860) and the subpath matrix (SD-2861) are
now the regression net for public exports, but the rule that PRs adding
public exports must keep them green was implicit. Make it explicit in
CONTRIBUTING.md so contributors know which gates apply and how to run
them, and add a checkbox to the "Before Submitting" list.

The enforcement is unchanged; this is the documentation that names it.

Author: caio-pizzol

CONTRIBUTING.md

@@ -226,6 +226,17 @@ pnpm run format   # Run Prettier
 
 ## Pull Request Process
 
+### Adding a public API
+
+If your PR adds a new public export from `superdoc` (a new `@typedef` in `packages/superdoc/src/index.js`, a new value re-exported from a public subpath, or a new subpath in `package.json` `exports`), it must ship with a type test that exercises the new surface under strict mode.
+
+The consumer matrix at `tests/consumer-typecheck/` is the regression net. Two automated checks enforce this on every PR:
+
+- **`check-public-types.mjs`** -- the assertion list in `tests/consumer-typecheck/src/all-public-types.ts` is auto-derived from the `@typedef` block in `packages/superdoc/src/index.js`. Adding a new typedef without regenerating the list fails CI. Run `npm run check:types:write` from inside `tests/consumer-typecheck/` (or `node tests/consumer-typecheck/check-public-types.mjs --write` from the repo root) and commit the regenerated file.
+- **`typecheck-matrix.mjs`** -- every typed public subpath in the RFC inventory has at least one matrix scenario. If you add a new subpath, add a fixture under `tests/consumer-typecheck/src/` and a corresponding entry in the matrix, and update the inventory in `docs/architecture/package-boundaries.md`.
+
+The point of these gates is to keep customer TypeScript builds working. A new export that ships without a type test can collapse to `any` (or fail to resolve) for consumers without the team noticing.
+
 ### Branch Naming
 
 - `feature/description` for new features
@@ -273,6 +284,7 @@ When you open a PR, the following checks run automatically:
 - [ ] Code is formatted (`pnpm run format:check`)
 - [ ] Commit messages follow conventional commits
 - [ ] PR description links to related issue
+- [ ] If you added a public API, the consumer-typecheck gates pass (see [Adding a public API](#adding-a-public-api))
 
 ## Release Process