Vendor the OMA Object catalogue into priv/ddf

[DominicFinn]

Problem

wick/priv/ddf/ contains three files: 3.xml, 6.xml, 3303.xml. The DDF parser
(Wick.Object.Ddf) works and is tested — it just has almost nothing to parse.

Eclipse Leshan bundles 19 standard models out of the box. A server cannot type a TLV
payload (#1), validate a Write, or render a useful Discover for an Object it has never
heard of.

Scope

• Vendor the OMA registry DDF XML for at least the core set: 0 (Security), 1
  (Server), 2 (Access Control), 3 (Device), 4 (Connectivity Monitoring), 5
  (Firmware Update), 6 (Location), 7 (Connectivity Statistics), 21 (OSCORE), plus
  the common IPSO objects already in use.
• Wick.Object.Registry.standard/0 returning a registry preloaded with the bundled set,
  built at compile time so there is no runtime file IO.
• Record provenance and licence for the vendored XML in priv/ddf/README.md — these come
  from the OMA LwM2M registry and should be attributed.
• Add the corresponding versions once Version-aware Object model lookup in Wick.Object.Registry #9 lands.

Acceptance

• Wick.Object.Registry.standard/0 resolves every bundled Object.
• The bundled models load at compile time; no priv/ read on the hot path.
• A refresh procedure is documented so the catalogue can be re-synced from upstream.

Good first issue — mostly mechanical, high value for the codecs.