First slice of the conversation-panel card unification. Adds the
category-3 collapser primitive (lib/widgets/): a list of 1..N inner item
cards, collapsed ticker <-> expanded framed inner canvas, with the agreed
chrome — color drives the border + chevron/label tint, a fixed-width
counter slot, the status icon hard against the right edge, chevron hard
against the left edge, background + caret toggle (D-78 tail-follow). The
aggregate status/count/echoed-title are caller-computed, so the widget
stays free of conversation semantics; inner items keep their own per-item
status.
Also adds clide_card_metrics.dart — shared spacing constants (gap, radius,
header padding, counter slot width) for all three card categories.
Not yet wired into the stream (no user-visible change); migration of the
group/tool cards follows. Widget test + golden included.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
jpmschweitzerandClaude Opus 4.8
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