docs: complete Arch prerequisites and scope device setup notes

Document matching kernel headers, pacman dependencies, repository checkout, mode-switch destination creation and MCU1 branch selection. Distinguish missing DKMS builds from stale module indexes and firmware-loader IDs from working interfaces. Correct the installer support claim and explain the NetworkManager connectivity penalty with its upstream reference.

Keeps the contributor-reported Lukton DQ-211 results scoped to that unit. Validation: all Bash examples parse, local links resolve, and commands were checked against the repository installer, Makefile, AUR packaging and upstream documentation. Follow-up corrections for #109; no Arch hardware test is claimed.

(cherry picked from commit 1b071be3e1)

INSTALL_SCRIPT.md

@@ -17,7 +17,7 @@ module without replacing the distribution's `btusb.ko`.
 - Wi-Fi-only and Wi-Fi/Bluetooth combo adapter support from the same branch
 - Comprehensive error handling
 - Colored output and logging
-- Compatible with Ubuntu, Debian, Fedora, and derivatives (DKMS supported `dkms.conf`)
+- Compatible with Ubuntu, Debian, Fedora, Arch Linux, and derivatives (DKMS supported `dkms.conf`)
 
 ## Usage