Center tagline bands, frame anchors evenly, fill to the window edge, and resize columns from their grip

Every tag band (workspace, column, pane) is centred in its row, so text sits
at one baseline offset. Anchors are inset from the column rule by that same
margin, so they are square and the band frames them on the left, top and bottom;
the pane mark moves to stay centred. When the window is not a whole number of
cells, the bands, rules, spines and scroll thumbs at the right and bottom edges
run on through the leftover pixels.

A column grip dropped short of another column's place now moves the column's
left edge, with a dashed rail preview, sharing setColumnPairWidth with the
border drag. A release still on the grip changes nothing, and a pair too
narrow for two MINW columns is left alone.

PARDES_TEST_PAD adds leftover pixels to a test-mode window and capture.

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