docs + doctor: be honest about amd64-only support

The README sold the product as "Linux with /dev/kvm"; the deeper docs
admit that the Makefile pins companion builds to GOARCH=amd64, the
kernel catalog ships only x86_64 entries, and OCI import pulls
linux/amd64 layers. arm64 users who show up through the README only
discover that after install fails in non-obvious ways.

Two surface-level fixes:

- README requirements list leads with "x86_64 / amd64 Linux — arm64 is
  not supported today", with a short note on the three places that
  assumption lives so users understand it's not a last-mile gap.
- `banger doctor` now runs an architecture check that passes on amd64
  and FAILS (not warns) on anything else, referencing the three
  downstream assumptions. Hard-fail rather than warn so a user on an
  arm64 machine doesn't waste time chasing unrelated preflight items.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -17,7 +17,11 @@ download); subsequent `vm run`s are seconds.
 
 ## Requirements
 
-- Linux with `/dev/kvm`
+- **x86_64 / amd64 Linux** — arm64 is not supported today. The companion
+  binaries, the published kernel catalog, and the OCI import path all
+  assume `linux/amd64`. `banger doctor` surfaces this as a failing
+  check on other architectures.
+- `/dev/kvm`
 - `sudo`
 - Firecracker on `PATH`, or `firecracker_bin` set in config
 - host tools checked by `banger doctor`