docs: reflect reality on drive filtering in iso/README

The "Drive list includes /dev/loop0 and /dev/sr0" rough-edge bullet
claimed the filter hadn't been added yet, but it has — `drives.py`'s
`parse_lsblk_output` skips everything with `TYPE != disk`, so loop
and rom devices never reach the picker. Tested.

Replaced with a note about the remaining real footgun: on bare-metal
installs, the USB stick the user booted from is `TYPE=disk` and would
show up alongside the actual install target, so a user could pick
their boot media by mistake. Not urgent while we test in VMs (the ISO
is a CD-ROM there, already filtered), but flagged so it's visible
when bare-metal testing starts.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

iso/README.md

@@ -50,4 +50,4 @@ mDNS (`proksi.local`) via avahi is installed but not yet wired. First milestone
 
 - **Disk space**: the first time you build on a fresh host, the squashfs/xorriso steps need ~15 GB free. If the host's LVM-root is smaller, `xorriso` silently dies at the very end with "Image size exceeds free space on media".
 - **No HTTPS yet**. The Furtka plan is "local CA + green padlock on `https://proksi.local`" — that's a later milestone. For now, plain HTTP.
-- **Drive list includes `/dev/loop0` and `/dev/sr0`**. `/dev/loop0` is the live ISO's own squashfs mounted in RAM; `/dev/sr0` is the CD-ROM itself. Both appear as install targets, which is wrong. Filter lives in `webinstaller/drives.py` and hasn't been added yet.
+- **Boot USB could appear as an install target on bare metal**. On a VM the ISO is a CD-ROM (filtered) and SATA is the only disk, so the picker only shows the install target. On bare metal with a USB stick, the USB is `TYPE=disk` and shows up alongside the real install drive; a user could in theory pick the USB they just booted from. Mitigating this needs detecting the boot media (via `findmnt /run/archiso/bootmnt` or similar) and filtering it out in `webinstaller/drives.py`.