fix(gtk): bundle the submenu arrow — Papirus has no pan-end-symbolic

05bab55's menu fix pointed -gtk-icon-source at the icon theme, but
Papirus doesn't ship pan-end-symbolic and its inheritance chain
(breeze-dark → hicolor) has no provider installed, so GTK menu submenu
arrows rendered as nothing (nm-applet: no cue that 'VPN Connections'
opens a submenu). Bundle the triangle as a store-path SVG recolored
via -gtk-recolor(url(…)) — the same mechanism adw-gtk3 uses for its
own check-mark assets, immune to icon-pack coverage.

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

modules/home/stylix.nix

@@ -35,6 +35,19 @@ let
     base0E = hex c.accentAlt;# magenta
     base0F = hex c.bad;      # brown/deprecated
   };
+
+  # Submenu arrow, bundled: adw-gtk3 (and our earlier fix) look up
+  # pan-end-symbolic in the ICON theme, but Papirus doesn't ship it and
+  # its inheritance chain (breeze-dark → hicolor) is broken here — so
+  # menu arrows rendered as nothing (nm-applet: no hint that "VPN
+  # Connections" opens a submenu). Same trick adw-gtk3 uses for its
+  # check marks: a bundled asset via -gtk-recolor(url(…)), immune to
+  # icon-pack coverage. Shape traced from Adwaita's pan-end-symbolic.
+  panEndSvg = pkgs.writeText "nomarchy-pan-end-symbolic.svg" ''
+    <svg xmlns="http://www.w3.org/2000/svg" width="16" height="16">
+      <path d="M 6 4 L 10 8 L 6 12 Z"/>
+    </svg>
+  '';
 in
 {
   config = lib.mkIf cfg.stylix.enable {
@@ -67,14 +80,16 @@ in
             menu menuitem:disabled, .menu menuitem:disabled {
               color: #${base16.base03};
             }
-            /* Submenu disclosure arrows — without this they pick muted. */
+            /* Submenu disclosure arrows — bundled asset, NOT an icon-theme
+               lookup: pan-end-symbolic is missing from Papirus and its
+               broken inheritance chain, so -gtk-icontheme() drew nothing. */
             menu menuitem arrow, .menu menuitem arrow,
             menu menuitem:hover arrow, menu menuitem:selected arrow {
               min-width: 16px;
               min-height: 16px;
               margin-left: 8px;
               color: inherit;
-              -gtk-icon-source: -gtk-icontheme("pan-end-symbolic");
+              -gtk-icon-source: -gtk-recolor(url("${panEndSvg}"));
             }
             menu menuitem:hover arrow, menu menuitem:selected arrow {
               color: #${base16.base00};