FixedPatchStepper(observe_window_exchange=True), which requires
observe_raw_exchange, allocates a second per-link FP64 ledger per body
level. While a window is open, the unchanged BlockAccumulateExchange
kernel also accumulates into it after each fine hop. Small API:
start_exchange_window() zeroes and opens at the current completed step;
read_exchange_window(body_id, level=) is non-destructive and reports
impulse with rates over the integer window duration. initialize/
_initialize_raw and step failure invalidate; remap builds a new instance
with no open window; Jet amplitude updates may stay in-window. Default
disabled: no allocation and no extra launch; last_substep/last_base_step
unchanged. summarize_exchange gains window='window' with duration.
Host contract tests (mocked driver) and a GPU gate against summed
per-base reads with a reordering-bound FP64 tolerance.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
