fix(server): resync a lagged consumer in-band instead of silently dropping VT

[phall1]

The last known corruption path from the TUI diagnosis (Rank 1). Closes the "stays mangled under heavy output" case.

Root cause

Each attached client has a per-pane output pump forwarding the actor's broadcast of VT chunks. The broadcast buffer is bounded (DEFAULT_OUTPUT_BROADCAST = 256); under a sustained output burst (a full-screen TUI repainting on attach/split, a log flood) a pump that falls behind gets RecvError::Lagged(n) — those n chunks are gone. The arm only warn!ed, so the client's libghostty mirror stayed permanently diverged until some unrelated resize/reattach happened to resync.

Fix

On Lagged, the pump asks the actor to broadcast an in-band resync — a full grid snapshot (PaneOutput::Resync) on the same ordered broadcast channel. Because it rides the same channel it lands after the post-lag tail and cleanly supersedes the gap:

• no double-apply — the snapshot is a full-grid reset (DECSTR + ED2 + replay), not an additive delta;
• no lost output — unlike a point-to-point snapshot request, which would race the still-buffered deltas and either double-apply or drop them.

Mechanism: a new ResizeRequest::resync_only flag tells the actor to skip the resize and only run its existing debounced resync broadcast (broadcast_resync, renamed from broadcast_resync_after_resize since it now serves both resize and lag recovery). The debounce coalesces a burst of lag events into one snapshot. try_send failure is benign (a resync is already queued, or the actor is gone).

Relationship to #132

#132 made a dropped-byte divergence self-heal on the next resync; this triggers that resync immediately on the drop instead of waiting for an unrelated event. Together they close the path.

Testing

• New resync_only_request_rebroadcasts_snapshot_without_resizing: the snapshot is re-broadcast and the grid size stays unchanged (the ignored 0×0 geometry must not take effect).
• Existing resize-resync + coalescing tests still pass.

just ci green.

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