web: time the MIDI out checks with a delay

[mishan]

Fixes the flaky MIDI clock goes out check in pagetest.mjs, which failed master's CI after #337 merged ("28 ticks 46.1 ms apart") and blocked that deploy. It had failed the same way on several PRs.

Cause

• The worklet stamps each MIDI message with the audio time of the frame it sounds at, and posts it to the page as it composes.
• At a delay of 0 (the page's default, and what the test used), a message that reaches the page a few milliseconds late is past due. handOff then sends it stamped now, i.e. Math.max(due, now).
• On real hardware the output latency covers this. Headless CI audio has almost none, so on a loaded runner the test measured the page's arrival times, not the piece's.
• The note checks in the same block were affected too, just less visibly (a half-beat note measured 479–492 ms).

Change

• The MIDI out block of the test runs with a 100 ms delay, which is what the setting is for.
• The delayed-note check keeps its own 200 ms, and the delay returns to 0 at the end.
• No change to the page itself.

Testing

• pagetest.mjs, 3 runs: the half-beat note now measures 500.0 ms exactly (it was 490–492 locally before), beat spacing is 1002.6–1002.8 ms, and the clock is 41.7 ms apart every time.
• I couldn't reproduce the CI failure under local CPU load, so the evidence is the code path above plus the exact note timing now.