Returns one row per party and instrument, with the position already summed. This is
the endpoint to use for an availability check or a balance enquiry — /token/holdings
returns the individual records that compose a position and leaves the arithmetic to
you, which is the right shape for audit and the wrong shape for "can this settle".

Three figures are reported and the invariant always holds:

total = locked + available

locked is reserved against in-flight settlement and cannot fund anything else.
available is the figure an availability check needs — a position total includes
locked amounts, so checking against total will oversell a position already
committed elsewhere.

holdingCount tells you how many records compose the position. A high count against
a small amount means a fragmented position, which matters because a transfer has to
consume whole holdings as inputs.

Positions are computed from open holdings at a single ledger offset, reported both on
the X-Ledger-Offset response header and as asOfOffset on each row. Two rows from
the same response are always mutually consistent.