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>

agent/BACKLOG.md

@@ -56,7 +56,6 @@ notify in a real session. Steps queued in HARDWARE-QUEUE. Close #76 on PASS.
   impermanence, BIOS/legacy boot.
 - **Boot-from-snapshot**: a systemd-boot equivalent of grub-btrfs.
 - **Night-light geo mode**: lat/long auto sunset/sunrise (means wlsunset).
-- **Per-theme icon overrides** / more icon packs (ROADMAP § Icon themes).
 - **MIPI/IPU software-ISP camera** support (no-UVC machines).
 - **VPN exit-node richer display** (country/city) (optional).
 - **NixOS release bump → v2** `[human]`: deliberate, hand-edited, never