Add per-axis scroll speed config for input devices (#2109)

* Add per-axis scroll speed config for input devices.

Accepts negative values to inverse scroll direction.

Properly complements/overrides global `scroll-direction` setting.

Includes docs and tests.

* Update per-axis scroll factor implementation after testing

- Refined configuration structure in niri-config
- Updated input handling to use per-axis scroll factors
- Added comprehensive test snapshots
- Updated documentation with per-axis examples

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Simplify per-axis scroll factor implementation per review feedback

- Make documentation concise and clear
- Remove unnecessary comments and test helper functions
- Use inline snapshots for tests
- Rename get_factors() to h_v_factors() for clarity
- Remove unnecessary .clone() calls (ScrollFactor is Copy)
- Reduce test count to essential cases only
- Fix comment about window factor override behavior

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Remove unnecessary ScrollFactor::new() helper function

The maintainer prefers minimal code, so removing this helper and
constructing ScrollFactor directly in tests.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix scroll factor behavior - all settings now multiply with window factor

Per maintainer feedback, both combined and per-axis scroll settings
should multiply with the window-specific scroll factor, not override it.
This ensures consistent behavior regardless of configuration method.

Also removed the now-unused has_per_axis_override() method.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Final cleanup: remove redundant comments and unused snapshot files

- Removed unused snapshot files (now using inline snapshots)
- Removed redundant inline comments in tests
- Simplified test descriptions to be more concise

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert scroll factor parsing tests to use assert_debug_snapshot

Updates parse_scroll_factor_combined, parse_scroll_factor_split, and
parse_scroll_factor_partial tests to use assert_debug_snapshot instead
of manual assert_eq comparisons, as requested in PR review.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert to inline snapshots as requested

- Convert all scroll factor parsing tests to use inline snapshots instead of external files
- Remove external snapshot files to keep test directory clean
- All tests still pass with inline snapshot assertions

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Fix missed assert_eq in parse_scroll_factor_mixed test

Converts the remaining assert_eq calls to assert_debug_snapshot
with inline snapshots in the mixed syntax test function.
Also fixes raw string delimiters from ### to #.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* Convert scroll_factor_h_v_factors test to use assert_debug_snapshot

Makes all scroll factor tests consistent by using snapshots instead
of assert_eq for better maintainability.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

* fixes

---------

Co-authored-by: Bernardo Kuri <github@bkuri.com>
Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Ivan Molodetskikh <yalterz@gmail.com>

docs/wiki/Configuration:-Input.md

@@ -37,6 +37,7 @@ input {
         // accel-speed 0.2
         // accel-profile "flat"
         // scroll-factor 1.0
+        // scroll-factor vertical=1.0 horizontal=-2.0
         // scroll-method "two-finger"
         // scroll-button 273
         // scroll-button-lock
@@ -53,6 +54,7 @@ input {
         // accel-speed 0.2
         // accel-profile "flat"
         // scroll-factor 1.0
+        // scroll-factor vertical=1.0 horizontal=-2.0
         // scroll-method "no-scroll"
         // scroll-button 273
         // scroll-button-lock
@@ -252,6 +254,8 @@ Settings specific to `touchpad` and `mouse`:
 
 - `scroll-factor`: <sup>Since: 0.1.10</sup> scales the scrolling speed by this value.
 
+    <sup>Since: next release</sup> You can also override horizontal and vertical scroll factor separately like so: `scroll-factor horizontal=2.0 vertical=-1.0`
+
 Settings specific to `tablet`s:
 
 - `calibration-matrix`: <sup>Since: 25.02</sup> set to six floating point numbers to change the calibration matrix. See the [`LIBINPUT_CALIBRATION_MATRIX` documentation](https://wayland.freedesktop.org/libinput/doc/latest/device-configuration-via-udev.html) for examples.