Say make check needs make bootstrap on a fresh clone (closes #282)

The third-party browser assets are not committed, so on a fresh clone
make check fails in the tests until make bootstrap (or make assets) has
fetched them. The Entrypoints section now says so up front, and why the
check does not fetch them itself: it must not change files in the repo.

Model: opus-5-5

README.md

@@ -1256,8 +1256,15 @@ standard: normalized scripts in `script/` are the entrypoints for the
 development workflow. Ten of the Makefile's seventeen targets are thin
 shims that call them; `build`, `run`, `dev`, `deps`, `clean`, `css` and
 `version` are inline commands with no script behind them, though
-`build` and `version` both take their value from `script/version`. We
-provide:
+`build` and `version` both take their value from `script/version`.
+
+`make check` needs the third-party browser assets in `static/`, which
+are not committed, so run `make bootstrap` (or just `make assets`) once
+after cloning. Without them the tests fail with a message naming that
+remedy. `make check` does not fetch them itself because it must not
+change any files in the repo.
+
+We provide:
 
 - `script/bootstrap` — install all dependencies (idempotent)
 - `script/setup` — make a fresh clone ready for development