feat(wifi-densepose-sar): coherent wideband RF tomography research crate (ADR-283)

New standalone leaf crate implementing the synthetic-aperture-radar
reconstruction primitive a handheld through-wall RF imaging device
would need: a stepped-frequency multi-position complex forward
measurement simulator, delay-and-sum backprojection reconstruction,
point-cloud extraction, and closed-form range/cross-range resolution
+ antenna-pose coherence-budget formulas checked against the
reconstruction's actual behavior in tests/physics_validation.rs.

Motivated by comparing this repo against Applied Electrodynamics'
"WaveSight" launch. Scoped explicitly below ADR-278's RISE/DiffRadar/
GeRaF reproduction gates: this is the bare measurement-model +
backprojection primitive, not a reproduction of any published system
or a claim about real hardware capability. Every number is
SYNTHETIC/L0 (ADR-282) -- no wideband RF hardware backs this crate.

24 tests (21 unit + 3 integration), 0 failed, clippy-clean. Adds a
tutorial walkthrough and MEASURED backprojection benchmark numbers.

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

@@ -0,0 +1,56 @@
+# ADR-283: `wifi-densepose-sar` — coherent wideband RF tomography research crate
+
+| Field | Value |
+|-------|-------|
+| **Status** | Accepted — implemented (P1) |
+| **Date** | 2026-07-30 |
+| **Parent** | ADR-278 (radar inverse rendering research program), ADR-282 (mandatory L0–L5 evidence ladder) |
+| **Relates to** | ADR-273/274 (`ruview-unified`'s `FmcwRadarCube` adapter, the eventual integration point), ADR-275 (`GaussianMap`, ditto) |
+
+## 0. PROOF discipline
+
+Every accuracy number this crate produces is **SYNTHETIC / evidence level L0** (ADR-282): generated by the crate's own forward simulator (`measurement::simulate_measurement`), scored against its own known ground truth (`ScatteringTarget` positions). Nothing here has been validated against real wideband RF hardware, and the crate contains no such hardware integration.
+
+## 1. Context
+
+A YC-backed company, Applied Electrodynamics ("WaveSight"), publicly launched a handheld "camera that can see through walls" using undisclosed radio-imaging technology. Comparing it against this repo's capabilities surfaced a real gap: `wifi-densepose-signal::ruvsense::tomography` implements *radio tomographic imaging* (Wilson & Patwari 2010) — RSS-based shadowing attenuation on a fixed-link topology, no coherent phase, no multi-frequency stepping, no synthetic aperture. It is a different technique from what a SAR-style through-wall imager needs: coherent, wideband, multi-position backprojection.
+
+`ruview-unified`'s `FmcwRadarCube` adapter (ADR-274) already normalizes wideband radar cubes into range profiles per position, and ADR-278 already names a radar-cube-output extension of the ADR-276 synthetic world generator as the intended sandbox for any future radar-inverse research. Neither, before this ADR, contained an actual backprojection reconstruction kernel — the primitive every candidate technique (matched-filter SAR, GPR imaging, RISE/DiffRadar-style inversion) is built on.
+
+## 2. Decision
+
+Ship `wifi-densepose-sar` as a standalone leaf crate (the `nvsim` pattern: pure Rust, deterministic ChaCha20 seeding, zero coupling to `wifi-densepose-hardware` or any real ingestion path) implementing:
+
+1. **Forward measurement model** (`measurement.rs`): simulates the complex, stepped-frequency returns a monostatic synthetic-aperture radar would record from known point scatterers — `y_{m,k} = Σ_j σ_j/R_{m,j}² · exp(-i·4π·f_k·R_{m,j}/c) + noise`.
+2. **Backprojection reconstruction** (`reconstruct.rs`): the matched-filter inverse of (1) onto a 3D voxel grid, parallelized over voxels (rayon).
+3. **Point-cloud extraction** (`pointcloud.rs`): threshold + local-maximum extraction from the dense voxel image.
+4. **Closed-form resolution/coherence formulas** (`resolution.rs`): `ΔR = c/2B` (range resolution), `δ_CR ≈ λR/2L` (cross-range/synthetic-aperture resolution), `Δp ≤ λ/8` (antenna-pose coherence budget, derived from a quarter-wavelength round-trip-path tolerance) — checked against the reconstruction's actual behavior in `tests/physics_validation.rs`, not merely documented.
+
+This is deliberately scoped **one level below** ADR-278's RISE/DiffRadar/GeRaF reproduction program: it is the bare measurement-model + backprojection primitive, not a reproduction of any specific published system, and not a claim about Applied Electrodynamics' undisclosed product (their waveform, antenna count, bandwidth, and algorithm are unknown; this crate applies the same well-established SAR/GPR physics — see Skolnik, *Radar Handbook* — to synthetic data).
+
+## 3. What this explicitly is NOT
+
+- Not a hardware driver. No VNA/SDR/wideband-RF-frontend code exists anywhere in this crate or was added to `wifi-densepose-hardware`.
+- Not wired into `ruview-unified`'s `FmcwRadarCube` adapter or `GaussianMap`. That integration is real future work (§5), deliberately deferred so the reconstruction physics validates in isolation first — the same staging ADR-278 §2.3 already prescribes ("sandbox-first... before hardware").
+- Not a reproduction of RISE, DiffRadar, or GeRaF. ADR-278's gates (G1–G4) are untouched by this ADR.
+- Not a real-world through-wall imaging performance claim. The forward model is free-space propagation only — no multipath, no per-material attenuation, no antenna gain pattern, no receiver noise figure. Real-world performance depends on all of these.
+
+## 4. Simplifications (honesty boundary)
+
+- **Monostatic, not MIMO.** A single antenna acts as both transmitter and receiver at each synthetic-aperture position (the standard stripmap-SAR simplification), not a multi-element MIMO array. Extending to bistatic/MIMO `(m, n)` transmitter/receiver pairs is straightforward given the existing measurement-model structure but not implemented.
+- **Isotropic antenna, no gain pattern.** Every antenna position radiates/receives equally in all directions.
+- **Free-space propagation only.** No multipath, no material transmission/reflection/attenuation (contrast `ruview-unified::synth::room`'s Fresnel material model, which is narrowband-CW and not yet extended to wideband — a natural follow-up, §5).
+- **`1/R²` two-way amplitude falloff, no calibration.** Real receivers have finite dynamic range, noise figures, and require calibration against a known reference target; none of that is modeled.
+
+## 5. Follow-up (not in this ADR's scope)
+
+1. Extend `ruview-unified::synth::room`'s image-method ray tracer to emit wideband stepped-frequency multi-position cubes (per ADR-278 §2.3), and wire `wifi-densepose-sar::reconstruct` against that richer (multipath-aware) synthetic generator instead of the free-space-only model here.
+2. A `ruview-unified` integration adapter converting `ReflectivityImage`/`PointCloudPoint` output into `RfGaussian`/`GaussianMap` primitives (ADR-278 §2.4's stated integration contract).
+3. Bistatic/MIMO measurement model.
+4. Any of ADR-278's actual gated reproductions (RISE first), if and when that program proceeds — this crate would be a component, not a substitute.
+
+## 6. Consequences
+
+- 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).