Clarify register diagrams and R_UID semantics#225
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Add a note describing how register layout diagrams are oriented: single-value registers most significant first, structured registers byte 0 first, with little-endian wire order. Remove the single-cell diagrams from R_UID and R_TAG. Redefine R_UID as an opaque identifier with no numeric interpretation, optional to implement and, when implemented, unique among devices that share the same R_WHO_AM_I. Drop the little-endian byte-order claim from R_TAG, and remove the R_SERIAL_NUMBER derivation from R_UID.
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Resolves the inconsistency in how core register byte and bit layouts are documented, and corrects the byte-order description of the identifier registers.
Register diagrams previously mixed orientations without explanation. The integer and bitfield registers are drawn most-significant-first,
R_VERSIONis drawn in field order, andR_UIDandR_TAGcarried single-cell diagrams that conveyed nothing beyond their length. This adds a short convention describing the two orientations and the little-endian wire order, and removes the two uninformative diagrams.It also corrects the description of core device identifier registers.
R_UIDwas described as a 128-bit little-endian value, but neither the Pico nor the ATxmega core treats it as a number; both store opaque bytes.R_UIDis now defined as an opaque identifier that is optional to implement and, when implemented, MUST be unique among devices sharing the sameR_WHO_AM_I, so that the two registers together uniquely identify a device. Little-endian wording was dropped fromR_TAG, and theR_SERIAL_NUMBERdeprecation note no longer claims to duplicate the first two bytes ofR_UID.Closes #201