Viewer polish: full-message overlay rebuild churn + widget conversation tail

[rz1989s]

Summary

Viewer/UX polish deferred from SPEC-6-4 (PR #82):

1. Full-message overlay rebuild churn during live appends — while the full-message overlay is open on a live timeline, every live append triggers renderShell(), which rebuilds the overlay stack (src/panel/fleet-panel.ts live-subscription callback). Content comes from the stable this.fullMessageEvent, so behavior is correct — but an early-return in the render path when fullMessageEvent is set (and its event is unchanged) avoids the churn.
2. Widget conversation tail (original SPEC-6-4 brainstorm garnish) — the above-editor widget could show the last N lines of the selected running run's conversation, consuming the same public fine-tier events (fleet:child:* / RunLog.subscribe). Only worth doing when a consumer asks; the live overlay covers the need today.

Both are additive polish; no contract impact.

Surfaced during the SPEC-6-4 SDD run (dogfooding).

[rz1989s]

Part 1 (overlay rebuild churn) — shipped in PR #99 (main @ eb0541f): the live-append subscription now skips renderShell() while the full-message overlay is open. Overlay scroll/selection survive appends; data stays current; closing the overlay picks up everything appended underneath. Behavioral tests: real RunLog + headless FleetPanel, including the negative case (tail-follow still rebuilds when the overlay is closed).

Part 2 (widget conversation tail) — closing as won't-fix-until-a-consumer-asks, per this issue's own note: "Only worth doing when a consumer asks; the live overlay covers the need today." Nothing has changed that assessment: no consumer has asked, and the live timeline overlay + (now non-churning) full-message overlay cover the need. Re-file if a real consumer surfaces.