docs(webcam): tuning tools in the app-suite template + libcamera gap filed

- templates/downstream/home.nix: commented "Webcam tuning" entries
  (cameractrls, v4l-utils) — bare packages, so template not toggle,
  per the option-surface rule; rarely needed since the IR-hide covers
  the common dual-sensor case.
- agent/BACKLOG.md PROPOSED: the portal/Flatpak camera picker still
  lists the internal IR sensor (libcamera path, untouched by the v4l2
  rule) — three options laid out, recommending document-only until a
  T14s-style machine is available to test a libcamera-side rule.

Closes BACKLOG NOW#7. Verified: V0 (flake check green; template still
evaluates through mkFlake).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

templates/downstream/home.nix

@@ -113,6 +113,11 @@
     # kdePackages.kdenlive        # video editor (pulls in KDE libs)
     # handbrake                   # transcoder
 
+    # Webcam tuning (rarely needed — the distro's IR-hide covers the
+    # common dual-sensor problem; these are for genuine tuning cases)
+    # cameractrls                 # GUI for exposure/focus/zoom, saves per-camera presets
+    # v4l-utils                   # v4l2-ctl CLI: list formats, poke controls directly
+
     # Media
     # vlc
     # spotify                     # (unfree)