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/JOURNAL.md

@@ -17,6 +17,27 @@ Template:
 
 ---
 
+## 2026-07-10 — Icon themes: opt-in cross-pack overrides (no default bloat)
+- **Task:** LATER "per-theme icon overrides / more icon packs". Recon found
+  papirus is already ~1 GiB and ships only Papirus/Dark/Light (no free
+  variety); real variety needs extra packs at ~hundreds of MB each. Bernardo
+  chose the **opt-in, no-default-bloat** shape (over shipping packs).
+- **Did:** `theme.nix` resolves `nomarchy.theme.iconThemePackage` from a
+  small `iconPacks` registry (papirus·tela·qogir·reversal·numix, by name
+  prefix). Default themes name `Papirus-*` → resolves to the *plain* papirus
+  derivation (no symlinkJoin, identical closure); only a theme naming another
+  pack's set symlinkJoins that one pack in. `stylix.nix` uses it
+  (mkDefault). Opt-in note in `templates/downstream/home.nix`; ROADMAP §
+  Icon themes updated (incl. the deliberate no-default-packs decision).
+- **Verified:** **V1** — HM activationPackage builds (exit 0). Eval proofs:
+  default `iconThemePackage` = plain `papirus-icon-theme-*` (zero delta);
+  with `icons="Tela-dark"` → `nomarchy-icon-themes` symlinkJoin (papirus+tela)
+  and name flows, **without building tela**. All 5 pack attrs exist in the
+  pinned nixpkgs. `template-sot` + `option-docs` + `flake check` green.
+- **Pending:** none. No default visual change (identical papirus), so no
+  screenshot needed; the opt-in render is the downstream user's to see.
+- **Next suggestion:** NEXT still #76 `[blocked:hw]`; LATER/QA-sweep next.
+
 ## 2026-07-10 — #76 slice 3: hibernate VM test (V2 PASS) → #76 agent-complete
 - **Task:** BACKLOG #76 slice 3 — the mandatory V2 hibernate test + V3 queue.
 - **Did:** Added `checks.hibernate` (flake.nix), modeled on nixpkgs'