[cpp-RCP-03] MDIO endpoint uses an invented Clause 22/45 PHY-address model instead of the spec's addressing scheme #72

Location: include/rcp/mdio.hpp — MdioClause enum, MdioRequest struct (phy_address/device_or_reg/register_address), kMaxPhyAddress range check (~lines 45-97).

Finding: The MDIO endpoint here is modeled entirely around an IEEE 802.3 Clause-22/Clause-45-style PHY-register access scheme: a 5-bit PHY address, a device-or-register field, and a clause selector. The governing specification's actual MDIO endpoint addressing scheme is different — it's keyed by a small mode selector (single/multiple word access to an MMD or MMS), a device/module address, and a payload whose width depends on which of those modes is selected, following the addressing convention referenced from the relevant OPEN Alliance SPI companion spec rather than a raw Clause-22/45 PHY address. There is no PHY-address or Clause-22/45 concept in the governing spec's MDIO section at all, so the whole address model implemented here is a different scheme, not merely a narrower encoding of the same one.

Recommendation: Rebuild the MDIO request/response types around the spec's actual mode/address/payload fields (a small mode selector, an address field sized per the referenced addressing convention, and a payload whose width is selected by the mode), and remove the Clause-22/45 abstraction — it doesn't correspond to anything the spec defines for this endpoint.

Verification: Independently re-read include/rcp/mdio.hpp against current HEAD (commit 4b9449a, 2026-07-29), and cross-checked against the governing specification's MDIO endpoint section, which documents a mode-selector + address + variable-width-payload scheme with no PHY-address or clause concept. Confirmed the divergence.

Filed from the 2026-07-29 ecosystem audit register; independently re-verified against current HEAD before filing.