refactor: implement component-based architecture for enhanced maintainability

- Reorganize directory structure into core/, features/, and themes/
- Colocate application Nix logic, configs, scripts, and theme overrides
- Implement 'Inversion of Control' for theming: apps now pull theme-specific layouts
- Update flake.nix and shared library paths to match the new structure
- Document the new Feature-Centric architecture in README.md

themes/engine/stylix-compat.nix

@@ -0,0 +1,35 @@
+{ config, lib, ... }:
+
+# Compatibility shims for stylix target modules
+# Stylix unconditionally imports all target modules, which expect certain
+# program options to exist. This module defines stub options for programs
+# we don't use to prevent evaluation errors.
+
+{
+  options = {
+    # Neovim: stylix uses initLua but home-manager uses extraLuaConfig
+    programs.neovim.initLua = lib.mkOption {
+      type = lib.types.lines;
+      default = "";
+      description = "Lua code to run at init (compatibility shim for stylix)";
+    };
+
+    # OpenCode: stylix expects programs.opencode options
+    programs.opencode = {
+      tui = lib.mkOption {
+        type = lib.types.attrsOf lib.types.anything;
+        default = {};
+        description = "OpenCode TUI settings (stylix compatibility shim)";
+      };
+      themes = lib.mkOption {
+        type = lib.types.attrsOf lib.types.anything;
+        default = {};
+        description = "OpenCode themes (stylix compatibility shim)";
+      };
+    };
+  };
+
+  config = lib.mkIf (config.programs.neovim.initLua != "") {
+    programs.neovim.extraLuaConfig = config.programs.neovim.initLua;
+  };
+}