feat(engine,registry): SweepPoint carries RunReport; add aura-registry (cycle D / iter 2)

Iteration 2 of the run-registry cycle (#33): make the sweep family
self-describing and add the registry store.

Engine. SweepPoint.metrics (RunMetrics) becomes SweepPoint.report (RunReport),
and the sweep closure bound is now Fn(&[Scalar]) -> RunReport. This closes the
manifest-per-point gap cycle C explicitly deferred to #33 — each swept point now
carries its full (manifest, metrics), the unit the registry indexes (C18). All
engine-internal callers, the engine test fixture run_point, and the three sweep
tests were rethreaded in the same iteration so the crate stays green; the CLI
caller followed in the same change so `cargo test --workspace` is green again.

CLI. The sweep_report closure builds a per-point RunReport (manifest params =
param_space names zipped onto the point via scalar_as_param_f64; commit/window/
seed/broker as run_sample builds them) and prints via RunReport::to_json. The
hand-rolled sweep_point_to_json/json_string/scalar_token are retired (the report
renders itself), and the orphaned ParamSpec/SweepPoint imports dropped. Net:
aura run, aura sweep, and (next iteration) aura runs all print the same
RunReport shape. The two aura sweep goldens now pin the per-point manifest
params, NOT the commit (it is the real git HEAD, volatile).

Registry. New crate aura-registry: Registry::{open, append, load} over an
append-only runs/runs.jsonl (one serde_json line per RunReport), free rank_by
(best-first per metric — total_pips desc, max_drawdown/exposure_sign_flips asc),
and RegistryError (Io / Parse{line} / UnknownMetric). Five unit tests cover
append->load round-trip, missing-file = empty, corrupt-line = Parse{line},
per-metric ranking, and unknown-metric error. Built and unit-tested; NOT yet
CLI-wired (aura sweep persistence + aura runs list/rank are iteration 3).

Verified: cargo test --workspace green (aura-registry 5, aura-engine 93, aura-cli
7+8, others unchanged); clippy --workspace --all-targets -D warnings clean; aura
run stdout shape unchanged; aura sweep now emits full RunReports.

refs #33

Cargo.lock

@@ -83,6 +83,15 @@ dependencies = [
  "data-server",
 ]
 
+[[package]]
+name = "aura-registry"
+version = "0.1.0"
+dependencies = [
+ "aura-core",
+ "aura-engine",
+ "serde_json",
+]
+
 [[package]]
 name = "aura-std"
 version = "0.1.0"