BRO-001: render the hunk status/header line as a full-width banner

The shared hunk renderer draws a hunk's status/header line (the {ts, verb, uri} metadata row — dog/HUNK.h, currently a plain <date>\t<verb>\t<uri> in text mode) without a clear visual band. Make it a proper banner: abbreviated time + URI on a pale-yellow, full-width, black-text line. This is the shared renderer used by every projector (log/status/grep/diff/commit/blame), so the fix lands once and benefits all. Prerequisite for BLAME-005.

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