roots: fix the blob gutter's clipped click box and add vertical rhythm

The gutter’s .blob-nums a line-number link had no padding of its own, so its click/focus box hugged the glyph tighter than the row and could clip against the card’s overflow; add breathing room, a focus ring, and a pointer cursor. The blob table also sat flush against the card’s own top/bottom edge — pad the first and last row instead of the table itself, so the sticky gutter’s background stays unbroken.

Lands tr.sel selection styling ahead of the client-side script that will apply it, so the next commit is markup/behavior only.

fix: give .blob-nums a a real click box, focus ring, and pointer cursor feat: add vertical padding to the blob table’s first/last row only feat: add tr.sel selection styling for the upcoming line-selection script Assisted-by: Claude:claude-sonnet-5