♻️ Run doc tests outside the coverage gate

An example that exercises a line no hand-written test reaches would let
the `--100` gate pass on documentation alone. Node has no file-level
exclusion, and c8's `--exclude` only filters the report, so run the two
in separate processes: `test:coverage` runs c8 over the tracked tests
only (`git ls-files`, which skips the untracked generated files), and
`test:doc` runs the generated examples without c8. `test:ci` chains
them.

CONTRIBUTING.md

@@ -140,8 +140,9 @@ CI step. Pick the prefix that matches the script's lifecycle:
   `npm run fix`; the diff is the review surface.
 - `test:*` — test scripts. `test` is the canonical entry point (`check:tsc` +
   unit tests); `test:unit` skips the typecheck for fast local iteration;
-  `test:ci` runs the suite under c8 and fails below 100% coverage on `src/`
-  (CI-only; `verify` stays coverage-free).
+  `test:coverage` runs c8 over the hand-written tests only; `test:doc` runs the
+  generated doc examples without coverage; `test:ci` chains the two and fails
+  below 100% coverage on `src/` (CI-only; `verify` stays coverage-free).
 - `watch:*` — long-running watchers for the manual inner dev loop. Aggregated by
   `watch`.
 - `maintain:*` — advisory repo-maintenance scans: read-only, but whole-project