docs(testing): describe the browser project and its helper

Say which claims belong in *.browser.test.tsx, how to run the project
alone, and what tests/browser/ provides. The browser install guard now
names the browser project beside the hero-geometry check, since both
launch the headless shell the CI slices install.

src/__tests__/unit-suite-browser-install-guard.test.ts

@@ -12,6 +12,12 @@
  * same suite: every dependency bump failed on a launch error that said nothing
  * about the bump. One workflow was fixed, one class was not.
  *
+ * Since v1.46 the suite also carries the `browser` project
+ * (`*.browser.test.tsx`, see `tests/browser/`), which launches the same
+ * headless shell through Vitest's Playwright provider for every interaction
+ * test. A slice without the browser now fails on many files rather than one;
+ * the install it needs is the same, so this guard covers both.
+ *
  * The sweep is textual and file-scoped, which is its limit: a workflow with two
  * jobs, only one of which installs the browser, satisfies it. It catches the
  * thing that actually went wrong — a whole workflow running the suite with no
@@ -58,8 +64,8 @@ describe("the browser the unit suite measures with", () => {
     expect(
       missing,
       "a workflow runs the unit suite without installing the headless " +
-        "Chromium the hero-geometry check measures with — that check refuses " +
-        "to skip under CI, so the job fails on a launch error instead",
+        "Chromium the hero-geometry check and the browser project launch — " +
+        "neither skips under CI, so the job fails on a launch error instead",
     ).toEqual([]);
   });
 });