Deliver subscribe overflow recovery as resized snapshot

[nwparker]

Summary

• send the binary terminal.subscribe pending-output-overflow recovery snapshot as kind: 'resized' instead of a second kind: 'scrollback' frame
• omit the output-byte seq from that wire frame (it is still used server-side for trimming covered pending chunks)
• skip the recovery snapshot entirely when the serialized snapshot has no output seq (renderer-source fallback), keeping the pre-existing bounded replay instead

Why

The shipped mobile client — the consumer of this stream — unconditionally drops a second scrollback event for an initialized handle, but applies a resized event carrying serialized by re-initializing xterm with fresh scrollback. With the old scrollback kind, the recovery snapshot introduced in #5025 was discarded client-side while the server had already trimmed the pending chunks it covered — making the visible gap strictly larger than the pre-#5025 behavior (which replayed the retained tail). Omitting seq also keeps output-byte sequences out of the client's layout-seq staleness filter.

For renderer-source snapshots there is no output seq, so covered chunks cannot be trimmed exactly — and under backpressure the renderer view may lag the queued chunks, so "serialized later" does not prove coverage. That path now keeps the bounded replay rather than applying an unverifiable snapshot.

Validation

• Updated recovery test asserts the recovery frame is kind: 'resized', reason: 'pending-output-overflow', with no seq on the wire; new test keeps bounded replay when overflow recovery has no output seq proves the seq-less path replays the retained tail with no recovery frame
• Both tests fail against the previous server behavior and pass with the fix
• pnpm exec vitest run --config config/vitest.config.ts src/main/runtime/rpc/terminal-subscribe-buffer.test.ts src/main/runtime/mobile-subscribe-integration.test.ts passed: 50 tests, 2 skipped
• pnpm exec oxlint on touched files passed
• pnpm run typecheck passed
• git diff --check passed

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[ghost]

Ready to review this PR? Stage has broken it down into 2 individual chapters for you:

	Title
1	Update overflow recovery to use resized frames
2	Verify recovery frame kind and sequence omission

_{Chapters generated by Stage for commit e4bbb86 on Jun 17, 2026 9:10am UTC.}

[nwparker]

Closing as superseded by merged PR #7214 (merge e84a8dd), which explicitly revived the complete term-speed-2 model/view chain into current main and then added newer scheduler, producer flow-control, snapshot, and reliability work. This old component/rollup branch no longer has an independent delta to rebase.