Keep the README platform table honest automatically

[trmxvibs]

Context

The README lists 17 backends with command, root handling and supported
operations, maintained by hand. The first person to add a backend without
touching the README makes the docs wrong — and nobody notices until a user
does.

Proposal

Add a test that reads the registry and asserts every registered backend
appears in the README table, and that no row exists for a backend that
isn't registered.

Acceptance criteria

• A test fails when a backend is registered but missing from the README
• A test fails when the README lists a backend that no longer exists
• CONTRIBUTING mentions it so the failure isn't a surprise

Pointers

• tests/backends/test_all_backends.py already iterates the registry —
  fits alongside it
• src/pkgwrap/backends/__init__.py — available_backends()

[strongdan]

I'll pick this one up if that's okay