tui: reduce mouse-wheel scroll to ~10% of page height per notch

[glemsom]

Fixes #655

Summary

Reduces the transcript history mouse-wheel scroll from height/3 per notch to height/10 (with a minimum of 1 row), making wheel navigation gentle and smooth. A physical wheel rotation emits several notch events, so previously a single flick moved ~3 pages.

Changes

• internal/tui/transcript.go: mouseWheelRows() now scrolls Height()/10 rows per notch (min 1); updated docs on navigateMouse.
• internal/tui/model_scroll_test.go: renamed TestScroll_mouseWheelMovesByThirdPage → TestScroll_mouseWheelMovesTenthOfPage, asserting the new ~10% amount.

Behaviour preserved (verified by existing tests)

• Wheel up breaks follow and scrolls toward older output; wheel down re-engages follow at the bottom
• Region routing unchanged: wheel over the history region scrolls, over the band/past-terminal-bottom does not
• Full internal/tui test suite passes

Note: internal/app and internal/engine test failures in this environment are pre-existing on main (require the bwrap binary which is not installed) and unrelated to this change.