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>

templates/downstream/home.nix

@@ -17,6 +17,13 @@
   # nomarchy.terminal = "kitty";        # swap the default terminal
   # nomarchy.waybar.enable = false;     # bring your own bar
   # nomarchy.stylix.enable = false;     # opt out of GTK/Qt theming
+  #
+  # Icon pack: Papirus ships by default (auto Dark/Light by theme mode) and
+  # is the ONLY icon pack in your closure. To use another, set the `icons`
+  # field in your theme-state.json to a theme name from a known pack — only
+  # that pack is then added (no bloat otherwise). Known packs: Papirus,
+  # Tela, Qogir, Reversal, Numix (see modules/home/theme.nix `iconPacks`).
+  # e.g. theme-state.json:  "icons": "Tela-dark"
 
   # ── Opt-in features — uncomment and tweak to enable ─────────────────
   # nomarchy.nightlight = {             # scheduled blue-light filter (hyprsunset)