feat(template): ship Chromium by default; record product decisions

- home.packages: chromium active; mime → chromium-browser.desktop
- Decisions: markdown docs stay in-repo; PPD default confirmed
- Formatter + zram still open (explained in session, not closed)

V0: flake check --no-build green.

docs/ROADMAP.md

@@ -287,7 +287,8 @@ Design/decision records and a running log of shipped work (items marked
   `libreoffice-fresh`, `vscode`, `gimp`, `inkscape` — ships *active* in the
   downstream `templates/downstream/home.nix` `home.packages`, with the heavier
   opt-ins (`texliveFull` — multi-GB; `texliveMedium` is lighter — plus
-  browser/email) commented just below. **Decision: no `nomarchy.apps.*` option
+  browser/email) commented just below — **Chromium is active by default**
+  (2026-07-10); other browsers stay commented. **Decision: no `nomarchy.apps.*` option
   surface.** Unlike `nomarchy.services.*` (real config behind each toggle —
   systemd units, subuid ranges, the Flathub oneshot), a bare package install
   has nothing behind the toggle: `mkIf x { environment.systemPackages = [p]; }`