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>

docs/ROADMAP.md

@@ -275,8 +275,17 @@ Design/decision records and a running log of shipped work (items marked
 - **Icon themes:** ✓ ships `papirus-icon-theme`; the resolved name lives on
   `nomarchy.theme.iconTheme` (the JSON's optional `icons` field, else
   Papirus-Dark/Light by `mode`) and feeds both Stylix's `gtk.iconTheme`
-  (Thunar/GTK apps) and rofi's `show-icons`. Remaining (optional): per-theme
-  `icons` overrides for the presets, or shipping more icon packs.
+  (Thunar/GTK apps) and rofi's `show-icons`.
+  ✓ **Opt-in cross-pack overrides (no default bloat):** `theme.nix` now
+  resolves `nomarchy.theme.iconThemePackage` from a small pack registry
+  (`iconPacks`: Papirus·Tela·Qogir·Reversal·Numix). 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. Adding a pack = one `iconPacks` row.
+  **Deliberately NOT shipping more packs by default** (Bernardo 2026-07-10):
+  papirus alone is already ~1 GiB and each extra pack adds ~hundreds of MB,
+  so variety is opt-in per install, not a default closure cost.
 - **Nicer shell out of the box:** ✓ zsh is the default login shell, with a
   starship prompt themed from the JSON, autosuggestions + syntax
   highlighting, and modern-CLI ergonomics — `cat`→bat (theme "ansi", so it