Disclose table rails per track with grips and seam inserts

Only the pointed or caret row and column draw a grip, with a six-dot
handle. Inserts appear on their seam with a line across the table.
Targets grow to 24px, rails fade in, and row-rail tooltips sit outside
the rail.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>