Render border gradients instead of a flat first stop

The frame collapsed Hyprland's border gradient to m_colors.front(), so a
gradient col.active_border showed up as a single flat color.

Snapshot the gradient (stops + angle) into a new header-only ChromeGradient
and fill the cairo path with a linear gradient built from it:

- AxisFor() reproduces the border shader's quadrant-folding progress
  function (y*sin(a) + x*(1-sin(a)), not a true rotation) as a cairo axis,
  so the frame's sweep stays in step with the window border it wraps.
  Checked against a port of the shader across all 360 degrees: cardinal
  angles exact, worst case 0.003 of the sweep (the shader folds on literal
  1.57/3.14/4.71 where this uses pi).
- SampleAt() interpolates in OkLab, where the shader interpolates, with
  each segment subdivided into sampled cairo stops so cairo's own sRGB lerp
  tracks that curve.

active_color/inactive_color become gradient config values, taking the same
syntax as general:col.active_border. Single-color configs are unaffected.

The cross-focus fade (m_realBorderColorPrevious + fade progress, lerped
between two gradients in-shader) is still not reproduced.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -56,7 +56,7 @@ Config values live under `plugin:hyprchrome:*` and are declared in `src/GlobalSt
 - `enabled` (bool)
 - `extent` (int, px the border extends past the window edge)
 - `follow_hyprland_border_color` (bool) — when true (default), the border tracks Hyprland's own resolved active/inactive border color instead of `active_color`/`inactive_color`
-- `active_color` / `inactive_color` (ARGB) — used when `follow_hyprland_border_color` is false, or when Hyprland reports no border color at all
+- `active_color` / `inactive_color` (gradient — one or more colors + optional angle, same syntax as `general:col.active_border`) — used when `follow_hyprland_border_color` is false, or when Hyprland reports no border color at all
 - `titlebar_height` (int, px)
 - `titlebar_text_size` (int, px)
 - `titlebar_font` (string, passed to `cairo_select_font_face`)
@@ -80,12 +80,14 @@ Per-window decoration (`src/ChromeDecoration.hpp/.cpp`): implements `IHyprWindow
 
 Render-pass element (`src/ChromePassElement.hpp/.cpp`): an `EK_CUSTOM` pass element (`ChromePassElement::draw()`) that runs once per frame per window and:
 - Derives the corner chamfer live from the window's own `rounding()` (in device px, scaled by monitor scale) — so the border's cut corners track the window's rounding through config reloads, per-window rules, and animations.
-- When `follow_hyprland_border_color` is true, derives the fill color from the window's *own* resolved/animated `m_realBorderColor` (matching Hyprland's `general:col.active_border`/`col.inactive_border`, already focus-aware); otherwise (or if that gradient is empty) uses the plugin's own `active_color`/`inactive_color` config, picked by `g_pCompositor->isWindowActive(window)`. Gradients are collapsed to their first stop since the ring is a flat cairo fill, not a shader.
+- When `follow_hyprland_border_color` is true, derives the fill from the window's *own* resolved/animated `m_realBorderColor` (matching Hyprland's `general:col.active_border`/`col.inactive_border`, already focus-aware); otherwise (or if that gradient is empty) uses the plugin's own `active_color`/`inactive_color` config, picked by `g_pCompositor->isWindowActive(window)`. Either way it's snapshotted into a `ChromeGradient` (`src/ChromeGradient.hpp`) — stops + angle — and filled as a cairo linear gradient across the whole texture. Note the cross-focus *fade* (`m_realBorderColorPrevious` + `m_borderFadeAnimationProgress`, which Hyprland's shader lerps between two gradients) is not reproduced; the frame snaps to the new gradient.
 - Measures the window title (via `GetTitleTexture`, which itself goes through Hyprland's own Pango-based `IHyprRenderer::renderText`) to size the title bar plateau to fit the text (clamped between `MinTitleBarWidth`/`MaxTitleBarWidth`), then requests the border texture at that plateau width.
 - Emits the border texture plus (if there's a title) the title texture as child `CTexPassElement`s.
 
 Geometry math (`src/ChromeDecorationGeometry.hpp`): a pure, header-only value type (`ChromeDecorationGeometry`) that computes every coordinate used by the cairo path in `GetBorderTexture` — outer/inner chamfered boundaries, the title bar plateau's flat run and "dip" back down to the normal top edge, the left/right edge inset notch, etc. `ComputeBase(...)` computes everything except the title bar's own width (which depends on measured text and is filled in afterward via `WithTitleBarWidth(...)`). This is the file to read/edit when changing the border's shape — `ChromeDecoration::GetBorderTexture` just walks cairo path commands using these precomputed coordinates.
 
+Gradient handling (`src/ChromeGradient.hpp`): a pure, header-only snapshot of a border gradient (stops + angle) with two things the cairo fill needs. `SampleAt()` interpolates in **OkLab**, because that's where Hyprland's border shader interpolates (it uploads `m_colorsOkLabA`) — `GetBorderTexture` subdivides each segment into `kStopsPerSegment` sampled cairo stops so cairo's own sRGB lerp between them tracks that curve. `AxisFor()` converts the angle into a cairo linear-gradient axis, reproducing the shader's quadrant-folding formula (`progress = y·sin(a) + x·(1-sin(a))`, which is *not* a true rotation) rather than a rotated axis, so the frame's sweep stays in step with the window border it wraps.
+
 Below `kFullSpanThresholdPx` (250px window width), `fullSpan` mode kicks in: there isn't room for the plateau + its dip back to the normal edge height, so the entire top edge stays flat at the reserved title-bar height instead. This is why some cairo path logic in `GetBorderTexture` branches on `geo.fullSpan`.
 
 `Common.hpp` provides the `Config::Values::C*Value` type aliases (`IntValue`/`BoolValue`/`ColorValue`/`StringValue`) and `RGBAToARGB()` — Hyprland's raw color ints are AARRGGBB, but config default literals are written in the more familiar RRGGBBAA and converted at compile time.