Add vertical scrolling for portrait outputs

[barrulus]

Portrait outputs can now scroll windows vertically while landscape outputs retain horizontal scrolling. Set main-axis globally, per output or per named workspace; existing windows update on config reload.

output "DP-2" {
    layout {
        main-axis "vertical"
        default-column-width { proportion 0.5; }
    }
}

Directional focus and movement keep their screen directions. Column-width presets control row height in vertical mode, and window-height presets control width within a row. Workspace navigation, gestures and overview placement follow the layout axes.

The implementation shares axis conversion between layout and input, reuses the existing config/sizing/animation APIs, and shares live/carousel placement. It preserves Biri’s hidden workspaces, sticky drag handling, multi-action binds and Shift-wheel carousel navigation. Regression fixes cover struts, client bounds, tab indicators, resize metadata, animation offsets and the pointer anchor when a window detaches during dragging.

Adapted from 0WD0’s reference implementation at 5b981892. Addresses the request in niri#1071; the design also incorporates the directional-shortcut feedback on niri#1677. Research and design details are in the implementation notes.

Validation:

• Default-feature full suite passed before the final cleanup: 336 compositor tests, 75 config tests, the wiki-config parser, 3 IPC tests and the IPC doctest. Headless tests used an empty input seat and software rendering.
• All 17 focused vertical-layout regressions passed; the detached drag-anchor regression was confirmed failing before its fix.
• 2,000 randomized layout cases passed, including orientation changes and output/workspace overrides.
• After cleanup, all 197 layout tests and 3 input-axis tests passed. Formatting, Clippy and the compositor build pass; Clippy reports existing warnings. Strict MkDocs build and binary config validation also passed.

Live portrait-monitor/touchpad testing and the GTK visual-test application remain outstanding.