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.

templates/downstream/home.nix

@@ -81,6 +81,7 @@
   home.packages = with pkgs; [
     libreoffice-fresh   # office suite (documents, sheets, slides)
     vscode              # code editor (unfree; allowUnfree is on)
+    chromium            # web browser (mime defaults → chromium-browser.desktop)
     gimp                # raster image editor
     inkscape            # vector graphics
     mpv                 # media player
@@ -88,9 +89,8 @@
     # pwvucontrol + calcurse ship with the Waybar module (volume right-click / clock)
 
     # ── More apps — uncomment to add ─────────────────────────────────
-    # Web browsers
+    # Web browsers (chromium ships above — swap/delete freely)
     # firefox
-    # chromium
     # ungoogled-chromium
     # brave                       # (unfree)
     # google-chrome               # (unfree)