fix(tui): disclose truncated tab titles

[kitlangton]

What

Keep truncated session tab titles stationary and disclose the full title in a delayed hover tooltip instead of scrolling it as a marquee.

The tooltip appears only when the title exceeds its available cells, stays within the terminal bounds, and works in both the vertical session rail and horizontal tab strip.

Before / After

Before: Hovering a truncated tab for 600 ms started a continuously looping marquee. The tab label changed beneath the pointer and could keep moving independently of the user scanning the tab strip.

After: The truncated title remains a stable, softly faded landmark. A deliberate 600 ms hover opens the full title beside or below the tab, and leaving, pressing, dragging, switching layout, or removing the tab dismisses it immediately.

How

• packages/tui/src/component/session-tabs.tsx replaces the marquee timer and offset state with overflow-only delayed disclosure state.
• Tooltip placement opens into available space for the vertical rail and anchors to the hovered tab in the horizontal strip, with terminal-bound fallback alignment.
• The retired marquee utility and its tests are removed; focused disclosure tests cover delay, overflow gating, and immediate dismissal.

Scope

This changes only pointer-hover disclosure for truncated TUI session tab titles. Tab sizing, keyboard navigation, close controls, drag ordering, and the web titlebar are unchanged.

Testing

• bun run test test/component/session-tabs-title-disclosure.test.ts test/context/session-tabs-model.test.ts from packages/tui (33 passing)
• bun typecheck from packages/tui
• Push hook: repository-wide bun turbo typecheck --concurrency=3 (34 successful tasks)
• OpenCode Drive storybook run in a real PTY at 100x40, verifying delayed hover disclosure and immediate dismissal in both orientations

Demo

Isolated OpenCode Drive session-tabs story using the production SessionTabs component. The pointer hovers a truncated title in the vertical rail, then repeats the interaction in the horizontal strip.

tab-title-tooltip.mp4