Add opt-in row-owned raw boundary exchange for blocked refinement

docs/development/raw_exchange.md

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+# Raw multilevel boundary exchange (opt-in candidate)
+
+Use `FixedPatchStepper(..., observe_raw_exchange=True)` and, after a successful
+base step, `read_raw_exchange(body_id, level=l, window="last_base_step")`.
+The alternative window is `"last_substep"`. Body IDs are object positions within
+the named level, not the object's original user label. The torque reference is
+that circle's geometry.center (the center used to construct BFL arms), reported
+in the root edge frame. A differing SimObject.center does not change this reference. Current geometry
+restrictions are unchanged: fixed complete bodies on the finest level, isolated
+from the transfer footprint. New observation currently requires blocked body
+storage; dense-body configurations raise before device allocation. Existing
+dense diagnostics are unchanged.
+
+Each result contains `wall`, `aperture`, `total`, with `force`, `torque`,
+`impulse`, `angular_impulse`, `mass_into_fluid`, and `mass_rate`. These are raw
+boundary momentum reaction, not validated stress or whole-nozzle mechanical
+load. No GI/advective correction or guessed thrust is added. Apertures retain
+their geometric classification when their prescribed velocity is zero.
+
+All results are base-lattice units per unit out-of-plane depth. For local
+spacing and timestep h=dt, raw `(jx,jy,tz,dm)` converts to impulse
+`(h*dt*jx,h*dt*jy,h*h*dt*tz)` and mass `h*h*dm`.
+Force/torque are impulse/angular impulse divided by the reported window length.
+`dm=b-a` is positive into the exterior fluid, while j is the opposite of the
+fluid's momentum change across that cut link. Interpolated BFL can introduce
+mass drift; dm is an observation, not an assertion of impermeability.
+
+`last_base_step` means [t-1,t] even after step(n); `last_substep` means [t-h,t]
+and is not a common instantaneous sample across levels. A Jet amplitude set
+before step(1) applies to all its substeps. Reads are repeatable; step(0) leaves
+them unchanged. Initialization and a remap candidate invalidate observation.
+Only successful stream synchronization publishes a window; failed instances
+reject reads rather than returning partially advanced measurements.
+
+Storage is one FP32 [N,4] raw array and one FP64 [N,4] integral per measured
+level, plus row metadata. No substep history is stored. Same-stream sequencing
+is halo/restriction -> BFL -> independent row accumulation -> step. There are
+no accumulation atomics or body obs offsets (blocked n_objects remains zero).
+Disabled observation allocates neither observation buffer and launches no
+accumulation kernel. The standalone periodic block caller passes explicit
+null optional arguments.
+
+## Gates
+
+CPU (standard library unittest, plus existing NumPy dependency):
+
+    PYTHONPATH=src python -m unittest discover -s tests/unit -p test_raw_exchange.py -v
+
+GPU, explicitly on a CUDA host with the existing environment:
+
+    PYTHONPATH=src python -m unittest discover -s tests/validation/multigrid -p test_raw_exchange_gpu.py -v
+
+The GPU suite includes an independent actual-population cut-link census and
+ACTIVE-fluid momentum/mass ledger with non-body boundary flux, observation
+on/off population equivalence, dense/blocked row comparison, three block
+partitions, zero/step/reversed/disabled Jet amplitudes, depth-four eight-sample
+accumulation, and a short-time whole-domain uniform-fine comparison with the
+same physical geometry, viscosity and Jet velocity. The uniform reference
+uses a test-only adapter to the standalone kernel's optional inputs.
+
+These are implementation checks. They do not establish physical Jet/stress
+accuracy, long-time AMR error, or convergence order. Resolution/long-time
+studies and independently validated continuum load benchmarks remain separate.
+GPU gates were prepared but not run in the authoring environment (no CUDA,
+PyCUDA or nvcc); run them before merging this candidate.