perf(web): stop replaying terminal buffers on rollover (#9707)

Keep bounded, byte-counted terminal chunks and append only unread output. Use UTF-16 cursors so compaction preserves live terminal replies. Reset on lifecycle changes or a real retained-data gap.

Keep the existing wire protocol, native buffer interface, and client retention limit. Native streaming and strict server replay byte bounds remain separate.

Continue the client helpers from https://github.com/pingdotgg/t3code/pull/9027 at source head 9391da2b48439d1d7a2b01d169e785682bf8abb8.

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 <noreply@anthropic.com>