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>

src/ChromeDecoration.cpp

@@ -27,6 +27,29 @@ std::vector<size_t> Utf8CodepointStarts(const std::string& s) {
   starts.push_back(s.size());
   return starts;
 }
+
+// cairo interpolates between color stops linearly in (premultiplied) sRGB,
+// Hyprland's border shader interpolates in OkLab - so rather than handing
+// cairo the gradient's own stops, each segment between them is subdivided
+// into this many sampled sub-stops. cairo's straight lines between those then
+// track the OkLab curve closely enough to be indistinguishable from the
+// compositor's own border, and it only costs anything on a cache miss.
+constexpr int kStopsPerSegment = 8;
+
+cairo_pattern_t* CreateGradientPattern(const ChromeGradient& gradient, double w, double h) {
+  const auto axis = gradient.AxisFor(w, h);
+  const auto pattern = cairo_pattern_create_linear(axis.x0, axis.y0, axis.x1, axis.y1);
+
+  // A single stop still needs two identical cairo stops to fill at all.
+  const int steps = std::max(1, (static_cast<int>(gradient.colors.size()) - 1) * kStopsPerSegment);
+  for (int i = 0; i <= steps; ++i) {
+    const double t = static_cast<double>(i) / steps;
+    const auto color = gradient.SampleAt(static_cast<float>(t));
+    cairo_pattern_add_color_stop_rgba(pattern, t, color.r, color.g, color.b, color.a);
+  }
+
+  return pattern;
+}
 }
 
 ChromeDecoration::ChromeDecoration(PHLWINDOW window) : IHyprWindowDecoration(window) {
@@ -91,12 +114,12 @@ uint64_t ChromeDecoration::getDecorationFlags() { return DECORATION_PART_OF_MAIN
 
 PHLWINDOW ChromeDecoration::GetOwner() { return windowRef.lock(); }
 
-SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx, float extentPx, float chamferPx, uint64_t colorValue, float titleBarHeightPx, float titleBarWidthPx) {
+SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx, float extentPx, float chamferPx, const ChromeGradient& gradient, float titleBarHeightPx, float titleBarWidthPx) {
   if (sizePx.x < 1 || sizePx.y < 1)
     return nullptr;
 
   if (cachedTexture && cachedTexture->ok() && cachedTexSize == sizePx &&
-      cachedExtent == extentPx && cachedChamfer == chamferPx && cachedColorValue == colorValue &&
+      cachedExtent == extentPx && cachedChamfer == chamferPx && cachedGradient == gradient &&
       cachedTitleBarHeight == titleBarHeightPx && cachedTitleBarWidth == titleBarWidthPx)
     return cachedTexture;
 
@@ -116,8 +139,6 @@ SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx,
   cairo_paint(cr);
   cairo_restore(cr);
 
-  const CHyprColor color{colorValue};
-
   cairo_set_fill_rule(cr, CAIRO_FILL_RULE_EVEN_ODD);
 
   // outer boundary, chamfered - the ring itself starts at geo.topPad, not 0,
@@ -196,8 +217,13 @@ SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx,
     cairo_close_path(cr);
   }
 
-  cairo_set_source_rgba(cr, color.r, color.g, color.b, color.a);
+  // The gradient's axis spans the whole texture (frame + title bar), not just
+  // the ring, so opposite sides of the frame land at opposite ends of it the
+  // way Hyprland's own border does.
+  const auto pattern = CreateGradientPattern(gradient, w, h);
+  cairo_set_source(cr, pattern);
   cairo_fill(cr);
+  cairo_pattern_destroy(pattern);
 
   cairo_surface_flush(surface);
 
@@ -205,7 +231,7 @@ SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx,
   cachedTexSize = sizePx;
   cachedExtent = extentPx;
   cachedChamfer = chamferPx;
-  cachedColorValue = colorValue;
+  cachedGradient = gradient;
   cachedTitleBarHeight = titleBarHeightPx;
   cachedTitleBarWidth = titleBarWidthPx;
 
@@ -215,18 +241,16 @@ SP<Render::ITexture> ChromeDecoration::GetBorderTexture(const Vector2D& sizePx,
   return cachedTexture;
 }
 
-SP<Render::ITexture> ChromeDecoration::GetTitleTexture(const std::string& title, float textSizePx, uint64_t colorValue, const std::string& fontFamily, int maxWidthPx, bool isActive) {
+SP<Render::ITexture> ChromeDecoration::GetTitleTexture(const std::string& title, float textSizePx, float alpha, const std::string& fontFamily, int maxWidthPx, bool isActive) {
   if (title.empty())
     return nullptr;
 
   if (cachedTitleTex && cachedTitleTex->ok() && cachedTitleTexTitle == title &&
-      cachedTitleTexSize == textSizePx && cachedTitleTexColor == colorValue &&
+      cachedTitleTexSize == textSizePx && cachedTitleTexAlpha == alpha &&
       cachedTitleTexFont == fontFamily && cachedTitleTexMaxWidth == maxWidthPx &&
       cachedTitleTexActive == isActive)
     return cachedTitleTex;
 
-  const CHyprColor borderColor{colorValue};
-  const float alpha = static_cast<float>(borderColor.a);
   const CHyprColor textColor = isActive
     ? CHyprColor{0.F, 0.F, 0.F, alpha}
     : CHyprColor{0xEE / 255.F, 0xEE / 255.F, 0xEE / 255.F, alpha};
@@ -262,7 +286,7 @@ SP<Render::ITexture> ChromeDecoration::GetTitleTexture(const std::string& title,
   cachedTitleTex = tex;
   cachedTitleTexTitle = title;
   cachedTitleTexSize = textSizePx;
-  cachedTitleTexColor = colorValue;
+  cachedTitleTexAlpha = alpha;
   cachedTitleTexFont = fontFamily;
   cachedTitleTexMaxWidth = maxWidthPx;
   cachedTitleTexActive = isActive;