fix: add stylix compatibility shims for missing program options

Stylix unconditionally imports all target modules, which expect certain
program options to exist even when those programs aren't enabled. This
causes evaluation errors.

Add stylix-compat.nix with stub options for:
- programs.neovim.initLua (maps to extraLuaConfig)
- programs.opencode.tui/themes

Also:
- Disable neovim/neovide stylix targets (we deploy theme lua via theme-loader)
- Set autoEnable = false to only enable explicitly listed targets
- Enable programs.neovim to satisfy stylix's neovim target
- Update stylix to latest version

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

flake.lock

@@ -428,11 +428,11 @@
         "tinted-zed": "tinted-zed"
       },
       "locked": {
-        "lastModified": 1775429060,
-        "narHash": "sha256-wbFF5cRxQOCzL/wHOKYm21t5AHPH2Lfp0mVPCOAvEoc=",
+        "lastModified": 1775927784,
+        "narHash": "sha256-9Gm33hvgcy4hR7O9EjWrw2kipyVBIHgNUtAHyWgLDhg=",
         "owner": "danth",
         "repo": "stylix",
-        "rev": "d27951a6539951d87f75cf0a7cda8a3a24016019",
+        "rev": "8f6e6b3844b3b0b59470e276bd3fe9ab04600081",
         "type": "github"
       },
       "original": {