docs(webcam): portal/Flatpak IR gap is v4l2-only (#71)

Ship the recommended document-only path: WirePlumber hideIrSensor acts
on the v4l2 monitor only; libcamera / xdg-desktop-portal / Flatpak
camera pickers can still list the internal IR sensor.

- HARDWARE.md §7: dual-sensor subsection (scope, workaround, deferred)
- ROADMAP § Webcam: ✓ #71; (b)/(c) stay hardware-gated
- README option row + HARDWARE-QUEUE T14s check for (b)/(c)
- Close BACKLOG #71; no libcamera rules

Verified: V0 (docs only).

agent/HARDWARE-QUEUE.md

@@ -377,3 +377,8 @@ QA machine), the **T14s** (webcam case).
       RGB+IR pair, bakes `hardware.camera.hideIrSensor`; `wpctl status`
       shows one colour source; an app picker lists one camera; Howdy-style
       direct `/dev/video2` reads still work.
+- [ ] **Portal/Flatpak libcamera IR (b)/(c)** — after #71 docs: on hardware,
+      confirm a Flatpak/portal picker still lists the internal IR node;
+      only then investigate (b) WirePlumber libcamera GREY-only monitor
+      rule or (c) libcamera/udev-layer hide. Do not ship either without a
+      live dual-sensor check (see HARDWARE.md §7 / ROADMAP § Webcam).