perf(wifi-densepose-sar): incremental phasor rotation in backprojection (~4.4-4.5x, MEASURED)

focus_at_point called Complex64::from_polar (a sin/cos pair) once per
(pose, frequency) term. FrequencySweep::frequencies() produces evenly
spaced frequencies by construction, so the per-term phase is an
arithmetic progression in the frequency index -- the phasor can be
evaluated once per pose and advanced by a fixed complex-multiply step
per frequency instead, turning K trig evaluations into 2.

focus_at_point's signature changes from a raw &[f64] frequency slice
to &FrequencySweep, so the evenly-spaced-frequencies precondition
this optimization depends on is a type-level invariant rather than a
caller-observed one -- an arbitrary non-uniform frequency list is no
longer constructible through this API at all.

MEASURED (criterion regression detection, p < 0.001): ~4.4-4.5x
faster across 512/4096/32768-voxel grids (300us/1.97ms/14.5ms vs the
prior 1.47ms/10.4ms/73.5ms). Proven equivalent, not just faster: a new
test independently reimplements the direct per-frequency computation
as a reference and checks the optimized path against it across four
sweep sizes (incl. the n_steps=1 degenerate case) and both on-target
and off-target points, to <1e-9 relative error.

25 tests (22 unit + 3 integration), 0 failed, clippy-clean.

docs/adr/ADR-283-coherent-wideband-rf-tomography-crate.md

@@ -53,4 +53,10 @@ This is deliberately scoped **one level below** ADR-278's RISE/DiffRadar/GeRaF r
 
 - The workspace gains a real (if intentionally scoped-down) coherent-imaging primitive where before there was none — useful groundwork for ADR-278 if that research program proceeds, and a direct, honest answer to "could this repo build a WaveSight-like device" (no, not without the hardware program described in the motivating comparison; yes, this is the reconstruction-algorithm groundwork such a program would need).
 - Zero risk to the existing `wifi-densepose-signal::ruvsense::tomography` (RSS-based RTI) code path or any production pipeline — this crate is not referenced by any of them.
-- 24 tests (21 unit + 3 integration physics-validation), 0 failed, clippy-clean. Criterion bench: MEASURED 512/4096/32768-voxel backprojection reconstruction throughput (see crate README for the numbers as last recorded).
+- 25 tests (22 unit + 3 integration physics-validation), 0 failed, clippy-clean. Criterion bench: MEASURED 512/4096/32768-voxel backprojection reconstruction throughput (see crate README for the numbers as last recorded). The incremental-phasor-rotation optimization (§7) cut reconstruction time ~4.4-4.5x, proven equivalent to the direct per-frequency computation it replaced.
+
+## 7. Follow-up optimization: incremental phasor rotation (2026-07-30, MEASURED)
+
+`focus_at_point` originally called `Complex64::from_polar` (one `sin`/`cos` pair) per (pose, frequency) term. Since [`FrequencySweep::frequencies`](../../v2/crates/wifi-densepose-sar/src/measurement.rs) produces evenly-spaced frequencies by construction, the per-term phase is an arithmetic progression in the frequency index — so the phasor can be evaluated once per pose and advanced by a fixed complex-multiply step per frequency, replacing K trig evaluations with 2. `focus_at_point`'s signature changed from a raw `&[f64]` frequency slice to `&FrequencySweep`, making the evenly-spaced-frequencies precondition this optimization depends on a type-level invariant rather than a caller-observed one.
+
+**MEASURED (criterion regression detection, p < 0.001): ~4.4-4.5x faster** across 512/4096/32768-voxel grids. **Proven equivalent**, not just faster: `reconstruct::tests::backprojection_incremental_rotation_matches_direct_per_frequency_computation` checks the optimized path against an independently reimplemented direct per-frequency reference, across four sweep sizes (including the `n_steps=1` degenerate case) and both on-target and off-target evaluation points, to <1e-9 relative error.