Show the frame rate the measured times allow in the overlay (#275)

The fps line of the performance overlay stops at the display's rate,
60 on most screens. It does not tell how much room a game has left.
The new top line answers that from the times the overlay measures.

- possibleFps in src/editor/frameMeter.ts is 1000 ms over the sum of
  the five areas, rounded down. It has no number while nothing took
  any time, and the overlay shows a dash then.
- The editor's share is in the sum, because it costs time while
  playing in the editor. A released game can do a little better.
- It is an upper bound. The GPU's time is not measured and not in it.
- e2e 59e reads the lines one further down and checks the new line
  against the times it measured itself.

Tests:
- Unit frame-meter: 11 passed (+2 for possibleFps)
- Gate: 197 files, 2855 tests, CHECK EXIT 0 (2853 + 2)
- e2e 59: ALL PASSED, 19 checks (+1), on melektaus-atx
- Counter-check: possibleFps without the editor's share fails the
  unit test
- The e2e total of 1602 is counted (1601 + 1), not measured

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -45,7 +45,7 @@ npm run build        # writes dist/ and regenerates index.html; the folder is wh
 The browser suite and the three example games need a machine with a GPU:
 
 ```bash
-node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1601 checks (GM_STEPS=20 for one step)
+node tools/seed-e2e.mjs && node tools/e2e.mjs      # the editor, 1602 checks (GM_STEPS=20 for one step)
                                                    #   every tool signs in first – see the Accounts page
 node tools/seed-jumprun.mjs && node tools/smoke-jumprun.mjs
 node tools/seed-tetris.mjs && node tools/smoke-tetris.mjs