fix(tests): drain view transition state between transition helper tests

[smalluban]

The transition helper suite in test/spec/html.js fails on WebKit with Expected spy warn to have been called 2 times. It was called 1 times., which currently blocks CI on unrelated pull requests. The should render component test starts a real view transition but only waits 200ms via resolveTimeout, so it never waits for that transition to settle. Since src/template/helpers/transition.js keeps the current transition in module scope as transition.instance, the unfinished transition leaks into the next test.

That leakage is what made the following test observe two warnings: with a transition still in progress, both rendered elements hit the "already in progress" branch. Without the leak the correct, deterministic result is one warning, because the first element starts the transition and only the second one hits that branch. Chrome and Firefox happened to still be mid-transition and passed by accident, while WebKit settles the transition faster and produced the real count.

This drains the pending transition in afterEach by awaiting html.transition.instance.finished, so each test starts from a clean state, and updates the expectation to the deterministic value of one warning. The assertion now verifies exactly what the test name describes rather than relying on cross-test state. No library source is changed.

Verified locally on Chrome (579/579 passing); WebKit headless does not launch on macOS, so CI covers that browser.

[coveralls]

coverage: 99.957%. remained the same — fix-transition-test-leak into main