scorecard: mirror the cvc::nav Track-1 base fields (Python parity, Track 2)

[transfix]

Track 2 of the nav-stats cross-repo build — the Python twin of the merged C++ cvc::nav base (transfix/libcvc #421–#424). Brings grl_snam.scorecard field-for-field and value-for-value in step, so a grl-snam eval and a cvcdbg dbg_arrival_check3 --episodes run report identical base scorecard numbers.

What lands (grl_snam/scorecard.py)

• NavCoverage dataclass (twin of nav_coverage) + compute_coverage reducer — plane-major indexing, a single broadcast truth plane, believed_free/phantom over the binary to_occupancy output (same p_thresh 0.5 / band 0.15 / optimistic contract).
• VehStats += convoy_id, formation_parent, slot_error_mean_m, formation_arrived, stall_steps, closest_approach_m, sense_flips, drive_steps, {alpha,beta,gamma,mu,mrisk,ext_force}_mean.
• EpisodeStats.coverage; NavScorecard += the Track-1 scorecard fields (formation rates, progress means, mean_coverage, mean_sense_flips, drive means) + to_dict.
• aggregate_nav reduction mirrors the C++ exactly: formation-mission convoy keying, coverage over all episodes, sense_flips/stall over all runs, closest over finite runs, drive means over drive_steps>0 runs.

Parity

test_scorecard.py += 7 cases pinned to the same hand-computed values as the C++ nav_stats_test feature-ON cases (formation 0.5/0.5/10.75; progress 4/2; coverage 0.625/0.25/0.625/0.25 → means 0.5; sense_flips 5; drive α2/β4/γ2/μ0.5/mrisk0.5/ext2). 11/11 pass, ruff clean.

An adversarial 3-lens cross-language review (aggregate / compute_coverage / corpus) confirmed 0-ULP parity — identical accumulators, divisors, and branches. The only constructible divergence needs an invalid convoy_id < 0 (unreachable from the collector; Python's dict is more robust than the C++ vector index) — now documented as a >= 0 contract. Stale cvc::dbg doc references corrected to cvc::nav.

The collector-layer twin (from_nav_stats seeding the new per-step fields + a formation_slot sampler hook) follows with the drive/training integration, mirroring the deferred C++ sim_world wiring.