refactor(config): give the tuned constants a real provenance, and make them reachable

Two problems, both of which made a number look more settled than it is.

The provenance was a dead link. config.hpp cited
docs/rep4-optimizer-results.md for prob_threshold, extinction_sec,
anneal_sec and the expansion default. That file was renamed to
model-bakeoff.md and then rewritten; the comments were never repointed,
so the most consequential constant in the pipeline appeared to have no
source at all.

Following it up produced something worse than a broken link.
prob_threshold=0.754 comes from the ORIGINAL rep4 document (still
readable at `git show d340da7:docs/rep4-optimizer-results.md`). The
rewrite that replaced it reports finding "a real scoring bug in
optimize.py: a candidate whose hardest film's replay timed out was
averaged over survivors instead of penalized, silently rewarding partial
coverage. Affected 3 of 16 training combos". So 0.754 was fitted under
scoring that was later found wrong, the corrected sweep converged
elsewhere, and no corrected prob_threshold is recorded anywhere. The
comment now says that, along with the surviving document's own verdict
that the optimum "generalizes unevenly -- strong on 3 of 5 held-out
films, badly broken on 2".

Four constants were unreachable. ownership_logodds lived on
TrackRegistry::Config, and max_views/admit_below/rho_max on
EvidenceDiscounter::Config, which main built with the one-argument
constructor -- so nothing short of a recompile could move any of them.
rho_max's own comment defers to "the sweep (VR-007)" for where it
belongs, and that sweep could not reach it.

They now live in Config with CLI flags and are exposed to the replay
harness. ownership_logodds is worth singling out: below it a track makes
no presence claim at all, so it decides whether an actor is reported
rather than how confidently -- arguably the most consequential constant
after prob_threshold, and until now unswept and unsettable.

No behaviour change: every default is the value that was compiled in.

TRACES: AR-025, AR-017 | SR-002

src/scene_preview.cpp

@@ -157,8 +157,13 @@ int main(int argc, char** argv) {
     auto same_person = same_person_probability(matcher_fn.calibration());
     TrackRegistry::Config reg_cfg;
     reg_cfg.track_extinction_sec = cfg.track_extinction_sec;
+    reg_cfg.ownership_logodds    = cfg.ownership_logodds;
+    EvidenceDiscounter::Config disc_cfg;
+    disc_cfg.max_views   = cfg.evidence_max_views;
+    disc_cfg.admit_below = cfg.evidence_admit_below;
+    disc_cfg.rho_max     = cfg.evidence_rho_max;
     auto registry = std::make_shared<TrackRegistry>(
-        reg_cfg, EvidenceDiscounter(same_person));
+        reg_cfg, EvidenceDiscounter(same_person, disc_cfg));
     matcher_fn.set_registry(registry);
 
     FaceTrackerFunc     ftracker_fn{cfg, registry, same_person};