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;
       };

modules/home/theme.nix

@@ -164,10 +164,43 @@ let
     if parsed.icons != "" then parsed.icons
     else if parsed.mode == "light" then "Papirus-Light"
     else "Papirus-Dark";
+
+  # Icon-pack resolution — opt-in, no default bloat. Papirus is the shipped
+  # default and the ONLY icon pack in the closure unless a theme's `icons`
+  # names a set from another pack; then that pack (and only it) is
+  # union-joined alongside papirus. Because the default themes name
+  # `Papirus-*`, the resolved package below is byte-identical to
+  # papirus-icon-theme (no symlinkJoin, no added MB). To offer another set:
+  # add one row to `iconPacks` and set `icons = "<Name>"` in a theme JSON
+  # (see templates/downstream/home.nix). Referencing `pkgs.*` here is
+  # eval-only — a pack enters the closure solely when it is the matched one.
+  papirusPkg = pkgs.papirus-icon-theme;
+  iconPacks = [
+    # pkg + the theme-name prefixes it provides (packs ship many named
+    # variants under one prefix, e.g. Tela / Tela-dark / Tela-blue).
+    { pkg = papirusPkg;                   prefixes = [ "Papirus" "breeze" ]; }
+    { pkg = pkgs.tela-icon-theme;         prefixes = [ "Tela" ]; }
+    { pkg = pkgs.qogir-icon-theme;        prefixes = [ "Qogir" ]; }
+    { pkg = pkgs.reversal-icon-theme;     prefixes = [ "Reversal" ]; }
+    { pkg = pkgs.numix-icon-theme-circle; prefixes = [ "Numix" ]; }
+  ];
+  # The pack whose prefix matches the resolved name (null = unknown name →
+  # GTK falls back gracefully; papirus is still present).
+  matchedPack =
+    let m = lib.findFirst
+      (p: lib.any (pre: lib.hasPrefix pre iconTheme) p.prefixes) null iconPacks;
+    in if m == null then null else m.pkg;
+  iconThemePackage =
+    if matchedPack == null || matchedPack == papirusPkg
+    then papirusPkg   # single pack → identical store path, zero closure delta
+    else pkgs.symlinkJoin {
+      name = "nomarchy-icon-themes";
+      paths = [ papirusPkg matchedPack ];
+    };
 in
 {
   config = {
-    nomarchy.theme = checked // { inherit iconTheme border; };
+    nomarchy.theme = checked // { inherit iconTheme border iconThemePackage; };
 
     # Feature toggles the menu writes (settings.nightlight.enable, …), exposed
     # alongside the appearance state. Feature modules mkDefault-read from here