ruview-unified: strengthen 3 tests whose names overclaim what they verify (PR #1437 follow-up)

Follow-up from the PR #1437 (ruview-unified, ADR-273..282) deep review: full write-up at https://gist.github.com/ruvnet/89795f3c4b8ea166cff5ac35ae4c7651

The functional bugs found in review are already fixed and merged (panics, NaN corruption, entity-conflation, wrong center-freq for real captures, missing task-field validation, untested purpose branches). This issue tracks three remaining items that are about test rigor, not correctness — none block anything, but the tests currently claim to verify more than they do:

1. heads.rs's factorized_pose_resists_room_shortcut_leakage never exercises the actual encoder. It hand-constructs synthetic 2-3 dim vectors where the anti-leakage property (room_x included vs excluded) is baked in by the test author, not learned or verified against RfEncoder::encode_content's real pooled-token representation. The PR's cited MPJPE numbers (0.0003 m vs 0.2534 m) describe the head's algebra on that toy input, not the real 24-dim-token -> 128-dim z pipeline. Worth an end-to-end version that trains the actual encoder + RePos head on synthetic multi-room windows and checks the room-shortcut leakage there.

2. gaussian/gain.rs's empty_map_returns_exact_friis never invokes the erf/Beer-Lambert path it's named for. With no Gaussians, optical_depth short-circuits to tau = 0.0 before the loop that would call line_integral/erf ever runs, so the test is really "float multiplication by an exact 1.0." The physically meaningful check (line_integral_matches_numeric_quadrature, 1e-6 vs 1D quadrature) is legitimate and already exists — this item is just about not citing the trivial test as if it validated the closed-form solution.

3. "Learns a 6.1 dB obstruction in 20 observations" is 20 repeats of one measurement converging one scalar unknown, since tau = occ * integral is exactly linear in the single spawned Gaussian's occupancy. It's a correct convergence check of the observe_link update rule (each call is a damped Newton step, residual shrinks geometrically), but reads as evidence of learning from diverse observations, which it isn't. Consider a version with several distinct links/measurements resolving genuinely different unknowns, if that scenario is meant to be demonstrated.

None of these are blocking — they're about making the test names and PR claims match what the tests actually exercise. Tracked on chore/pr1437-followup-test-rigor (currently empty, branched off main).