Pilot the PDU split by extracting MS-RDPRFX into `ironrdp-pdu-rfx`

We want to split `ironrdp-pdu` along Microsoft specification lines. RFX is a good pilot: self-contained codec PDUs, natural target for a future RFX codec crate (#1349), and doesn't touch the central RDPBCGR core.

## Goal

Define the per-PDU-crate template by shipping `ironrdp-pdu-rfx`. Patterns set here should be reusable for subsequent spec-family extractions without rework.

## Scope

- Move `crates/ironrdp-pdu/src/codecs/rfx/` (and related capability set entries) → `crates/ironrdp-pdu-rfx/`.
- Keep shared primitives in residual `ironrdp-pdu`.
- Update consumers (`ironrdp-graphics`, future RFX codec crates).