Derive the broker-independent position-event table (#114's PositionEvent rows)
from a completed PositionManagement run, as the first difference of the executed
book (deal = target - book - in_flight; in_flight = 0 for instant backtest
fills). A pure post-run reduction `derive_position_events(record, instrument_id)`
in aura-engine, sibling of summarize_r, reading the 14-column PM record
positionally; a reversal emits Close then the opposite open at one event_ts
(close first); a window-end open position emits its open with no synthetic Close.
Supersedes the rolled-back 0064 exposure-integral derive (abandoned per #117):
this derives from the executed book (the PM record), never from exposure deltas.
Stage-2 fixed-fractional / currency / equity-feedback sizing is out of scope
(#116 owns the equity->Sizer z^-1 register per C10).
Boss-signed: specify Step-5 grounding-check PASS (8/8 load-bearing assumptions
ratified by named green tests). Fork decisions recorded on #115.
refs #115
Brummel
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