Emit TerminalFrame::Diff for steady-state pacer ticks

[MasonRhodesDev]

Phase 8 of design 01. The daemon's per-tab Term task currently emits only TerminalFrame::Full at each pacer tick, paying one Arc<GridSnapshot> allocation per emit. Chatty TUIs (htop, animated progress bars, streaming agent output) hit this on the hot path.

Trigger: allocator pressure surfaced by a profile run on htop-class output, or an iroh-bandwidth complaint from a paired mobile peer.

Implementation sketch (recap from the design doc):

• Track previous viewport rows + cursor + mode in the Term task.
• On emit, if <= K rows changed since the last Full, build a Diff { rows_changed, cursor, mode, scroll_offset, bell }; otherwise re-emit Full.
• Periodic keyframe Full every N seconds (or every M seq) so a viewer that joined mid-stream picks up canonical state without a RequestFullFrame round-trip.
• Viewer side: LiveTerminalRuntime::ingest_frame's Diff arm currently logs and discards (app/src/terminal_runtime.rs ~line 400). Replace with a row-overlay path that mutates the cached live_snapshot.viewport in place, falls through to recompute_cached_snapshot.

Wire compatibility: TerminalFrame::Diff is already on the wire (daemon-proto/src/lib.rs). No protocol changes needed.

Surfaced from rc/terminal-hardening.