perf(web): stop rendering hidden terminals (#9718)

Stop post-construction terminal snapshots, canvas paint, and cursor timers while a drawer or right panel is hidden. Keep parsing output and answering VT queries, then render current state once on reveal.

Cover delayed WASM initialization, selection behavior, zero-size mounts, and reveal with real-core headless tests. Keep the existing startup background fill.

Continue the zero-size guard from source commit eb5b68103506b1bb5bd81c3b9e27f11e349743c9 in the terminal streaming contribution.

Created with GPT-6 Astra (preview) in Codex.

Co-Authored-By: Wout Stiens <71498452+StiensWout@users.noreply.github.com>
Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>