feat(theme): opt-in cross-pack icon overrides, no default bloat

theme.nix now resolves nomarchy.theme.iconThemePackage from a small
icon-pack registry (Papirus/Tela/Qogir/Reversal/Numix, matched by name
prefix). Default themes name Papirus-*, so the resolved package is the
plain papirus-icon-theme derivation — byte-identical closure, zero added
MB. Only when a theme's `icons` names a set from another pack is that
pack (and only it) symlinkJoined alongside papirus. stylix.nix consumes
it via mkDefault. Adding a pack is one iconPacks row.

Deliberately NOT shipping more packs by default: papirus alone is already
~1 GiB and each extra pack adds ~hundreds of MB, so variety is opt-in per
install (set `icons` in theme-state.json), not a default closure cost.
Documented in templates/downstream/home.nix + ROADMAP § Icon themes.

Verification: V1 (HM activationPackage builds). Eval proofs — default
resolves to plain papirus (zero delta); icons="Tela-dark" ->
nomarchy-icon-themes symlinkJoin (papirus+tela) with the name flowing,
without building tela; all 5 pack attrs exist in the pinned nixpkgs;
template-sot + option-docs + flake check green. No default visual change
(same papirus), so no screenshot; the opt-in render is downstream.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

modules/home/stylix.nix

@@ -60,10 +60,12 @@ in
       # GTK/file-manager/rofi icon theme, resolved from the JSON in
       # theme.nix (per-theme `icons`, else Papirus-Dark/Light by mode).
       # Stylix sets gtk.iconTheme from this; both dark/light point at the
-      # already-mode-resolved name.
+      # already-mode-resolved name. The package is papirus by default and
+      # only unions in another pack when a theme's `icons` names one
+      # (theme.nix — opt-in, no default closure bloat).
       icons = {
         enable = true;
-        package = lib.mkDefault pkgs.papirus-icon-theme;
+        package = lib.mkDefault t.iconThemePackage;
         dark = t.iconTheme;
         light = t.iconTheme;
       };