fix: unshrunk tabs cut their labels with an ellipsis #194

What

A tab whose bar has room for every label at full width still shows its label cut with an ellipsis. tabs.width rebuilds a tab's width as least + (natural - least) * fit, and with fit = 1 that sum is not exactly natural in floating point: it can come out a hair short. The label box is that width less the padding and close box, so it ends up a fraction of a pixel narrower than the label, and text.fit_box (#177), which checks width <= box exactly, cuts it. Every docked tab strip in octalide/ac shows "Behavi…" with room to spare.

Acceptance

• A tab that is not shrunk is exactly its natural width, so its label box is exactly its label's width
• A test checks the width equals the natural width for unshrunk tabs whose widths do not round-trip through the shrink formula