Bundle archiso initcpio hook directly in repo

Current archiso package does not install initcpio hook files (confirmed:
find /usr -path */initcpio/hooks/archiso returns nothing). Write the hook
and its mkinitcpio install script directly into the profile airootfs so
COPY carries them into the image without any package dependency.

Hook finds ISO by archisosearchuuid= kernel param, waits up to 30s for
the device to appear via blkid, mounts iso9660→squashfs→overlay onto
/new_root. mkinitcpio detects /new_root is already mounted and succeeds.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/profile/airootfs/usr/lib/initcpio/install/archiso

@@ -0,0 +1,19 @@
+#!/bin/bash
+# mkinitcpio install script for the archiso live-boot hook.
+
+build() {
+    add_runscript
+    add_binary blkid
+    add_binary mount
+    add_binary sleep
+    add_module squashfs
+    add_module overlay
+    add_module isofs
+}
+
+help() {
+    cat <<HELPEOF
+Locates the live ISO by UUID, mounts the squashfs airootfs, and sets up
+a RAM-backed overlay as the writable root filesystem.
+HELPEOF
+}