fix(tui): keep exact-fit tab titles stationary

[kitlangton]

What

Keep exact-fit session tab titles stationary and free of overflow fades when hover reveals the close button.

Before / After

Before

A title that exactly filled its resting tab width became one or two cells too wide only because hover reserved space for ×. That activated the marquee and its leading-edge fade, producing a dark mass at the front of an otherwise stationary-looking title.

After

Overflow is classified against the tab's resting title width. Exact-fit titles remain stationary and unfaded on hover; titles that already overflowed continue to marquee within the narrower hovered viewport.

How

• Add a strict display-cell overflow predicate in packages/tui/src/util/marquee.ts.
• Separate resting title width from the temporary hovered viewport in horizontal and vertical session tabs.
• Use resting width for marquee eligibility and overflow fades while retaining the hovered width for clipping and close-button layout.

Testing

• bun run test test/util/marquee.test.ts from packages/tui
• bun typecheck from packages/tui
• Pre-push bun turbo typecheck --concurrency=3