feat: horizontal mouse-wheel scrolling for the bufferline

[tusharmurali]

What

Adds opt-in horizontal mouse-wheel scrolling of the bufferline. When more buffers/tabs are open than fit on screen, scrolling the wheel while hovering over the bufferline shifts the visible window left/right to reveal off-screen buffers — without changing the active buffer (view-only scroll, like scrolling a horizontal list).

Enable it via:

require("bufferline").setup({
  options = {
    mouse_scroll = { enabled = true, step = 1 },
  },
})

Disabled by default, mirroring the existing hover option.

How it works

• A new lua/bufferline/scroll.lua registers <ScrollWheelUp/Down/Left/Right> expr-keymaps, mirroring hover.lua's <MouseMove> pattern. Each acts only when the mouse is over the tabline (screenrow == 1) and otherwise returns the original key, so scrolling over normal windows is unaffected. Both vertical and horizontal wheels drive horizontal movement (up/left = towards the start).
• The scroll position is persisted in state (scroll_offset) and honored by a parallel windowing path in the renderer (max_scroll_pivot + scroll_window in ui.lua). This leaves the existing, well-tested centring truncate logic untouched — it's only bypassed while actively scrolled.
• The view snaps back to auto-centring when the active buffer changes (a keyboard switch) or the buffer list grows/shrinks, so you're never stranded on a stale scroll position. When everything fits on screen, scrolling is a no-op.
• step controls how many buffers each wheel tick moves the view by.

Notes

• Works in both buffers and tabs mode.
• Requires mouse to be enabled. Delivery of <ScrollWheel*> events over the tabline row depends on the terminal/multiplexer (the same assumption the existing hover feature makes for <MouseMove>).

Tests

Adds unit tests in tests/ui_spec.lua covering the scroll-pivot clamping, window building/marker counts, and the handler's offset capture + clamping. Full suite passes and stylua is clean.