…n arch-test gate

`arch-test arm` probes ARMv5 EABI, divergent from kernel COMPAT (armhf
EABI v5+), and returns failure on modern aarch64 hosts (Ubuntu Noble
6.8 / Ampere Altra) with a fully working CONFIG_COMPAT path — routing
mmdebstrap through qemu-user emulation at ~10× slowdown. Replace with
a 4-tier detection: prefer the packaged `/usr/share/binfmts/qemu-arm`
descriptor if present, trust an already-enabled kernel registration,
otherwise probe COMPAT directly by exec'ing `ld-linux-armhf.so.3`,
falling back to the hand-rolled descriptor only when none applies.

Assisted-by: Claude:claude-opus-4.7

…registration

When CONFIG_COMPAT runs armhf binaries natively on aarch64 (≈10× faster
than qemu-user emulation), but qemu-arm is registered in kernel binfmt_misc,
the previous "trust existing setup" check returned early and the build
went through qemu anyway. Empirically: helios4 build 41 min via qemu vs
19 min native COMPAT on droid (RK3399).

Reorder probes:

1. Snapshot initial qemu-arm state.
2. COMPAT probe must run with qemu-arm OFF — kernel binfmt_misc routes
   armhf exec through qemu first when enabled, making the probe a liar
   about native capability. Temp-disable before probe; on probe failure
   (no COMPAT, e.g. Apple Silicon under HVF where AArch32@EL0 SIGILL's)
   restore qemu-arm and fall through to qemu setup. Toggle errors are
   not swallowed.
3. Trust-existing path validates via arch-test arm — the `enabled` flag
   alone does not attest that the interpreter actually runs (stale path,
   removed package); without this, broken registrations passed setup
   and failed later in chroot.
4. _verify_qemu_arm_executes helper called after each successful
   import/enable/repair branch, so qemu issues fail fast at host-prepare
   time instead of mid-rootfs-build.

The descriptor itself is left intact — only the active flag is toggled.

Gated by ARMBIAN_PREFER_NATIVE_ARMHF=no for explicit opt-out. Behaviour
preserved on hosts without COMPAT.

Assisted-by: Claude:claude-opus-4.7