Ports the radard lead-tracking work from the sunnypilot fork
(spicysapling/sunnypilot @ fix/lead-track-switching, through aa23300729)
into this Rivian fork. User-selectable via a new "Radar Lead Tracking"
cruise-panel selector (LeadTrackingMode, default = Stable Lead / Tier 2):

  - Tier 1 (Stock): stock per-frame greedy vision<->radar match.
  - Tier 2 (Stable Lead): lateral-weighted id hysteresis biases the held
    radar track, killing per-frame lead flicker where track ids are stable.
  - Tier 3 (Stable + Vision Coast): also holds the last-good lead through a
    sudden close/slow radar phantom the camera doesn't corroborate (chosen
    track-id changed, or radar lateral jump while vision held still), bounded
    by a 1 s cap, with a 40 m close-cut-in failsafe that never coasts past a
    near lead.

Rivian radar lateral correction: the opendbc Mando interface computes
yRel = 0.5 * -sin(az) * dist; the 0.5 over-compresses the lateral offset so
prob_y can't separate an adjacent-lane car from the in-path lead. Corrected
brand-gated at radar-point ingestion to a 0.70 effective scale (x1.4) so the
matcher, the stored Track, and the emitted lead all see the same lateral;
dRel/vRel untouched, non-Rivian brands are a no-op. Validated on the
sunnypilot corpus (tier-2 brake-jumps -4.7%, 0 gross-streak regressions).

Observability: new radarStateSP cereal message (leadOne/leadTwoCoasting)
drives an opt-in per-chevron coast tint + dev-UI COAST element, gated by
ShowVisionCoastOnChevron (default off).

Validated on macOS: 23 unit tests pass (test_radard_tiers.py), cereal and
params_pyx rebuilt, radarStateSP round-trips, new param keys resolve.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
(cherry picked from commit 9f0e77c)