feat: Gruvbox Dark theme for Yazi 26.9, icons included

Panel chrome, filetype colours and the full 725-entry icon table.

The icon table is Yazi's preset with every glyph colour remapped onto the
Gruvbox palette: nearest neighbour in Oklab with lightness weighted down so
hue drives the match, chromatic and greyish sources matched from separate
pools, and target tones below 3:1 contrast on #282828 excluded. 221 upstream
colours collapse onto 19 Gruvbox tones, dimmest icon at 3.48:1.

regen-icons.py reproduces the mapping against a newer Yazi preset.

Uses the current schema ([mgr]/[tabs]/[pick]/[cmp]), not the pre-25.x one.

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

THIRD-PARTY.md

@@ -0,0 +1,51 @@
+# Third-party material
+
+## Yazi icon data
+
+The `[icon]` block in `theme.toml` is derived from Yazi's built-in
+`theme-dark.toml` preset: the icon glyphs, file/extension/directory mappings
+and table structure are taken verbatim, only the `fg` colours were remapped.
+
+- Upstream: https://github.com/sxyazi/yazi — MIT License, Copyright (c) 2023 - sxyazi
+- Icon data originates from nvim-tree — MIT License, Copyright (c) 2023 nvim-tree
+  (see `LICENSE-ICONS` in the Yazi repository)
+
+Both are MIT-licensed; their notices are reproduced below.
+
+```
+MIT License
+
+Copyright (c) 2023 - sxyazi
+Copyright (c) 2023 nvim-tree
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in all
+copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+SOFTWARE.
+```
+
+## Gruvbox palette
+
+Colour values follow the Gruvbox palette by Pavel Pertsev:
+https://github.com/morhetz/gruvbox — MIT/X11 License.
+
+## Not included: syntax highlighting theme
+
+This repository deliberately ships **no** `.tmTheme` file. The Gruvbox
+`.tmTheme` commonly paired with Yazi comes from
+https://github.com/poperigby/gruvbox-dark-yazi, which is licensed **AGPL-3.0**.
+Bundling it would force AGPL-3.0 on this repository. Fetch it yourself if you
+want Gruvbox-coloured file previews — see the README.