refactor: registry deflation dispatch generic over the MetricVocabulary trait

The A1 cut of #147 item 2, part 1 of 2 (the substrate): the registry's
ranking/deflation machinery no longer bakes in the R metric vocabulary.

- aura-analysis gains the deliberately narrow MetricVocabulary trait (name
  resolution, roster, direction, value read, one null draw) and the
  single-sourced one_sided_p_laplace estimator; re-exported through
  aura-engine's existing seam (no Cargo.toml edge changes anywhere).
- aura-backtest now OWNS the R vocabulary: RunMetricKey (was the registry's
  private Metric enum, exposure_sign_flips alias preserved), r_based (the
  C10 classification), R_BASED_METRICS/RANKABLE_METRICS rosters, the
  centred moving-block null_draw, and member_metric_from_rs.
- aura-registry: metric_cmp/rank_by/null_best_of_k/member_sd generic over
  M: MetricVocabulary; optimize/optimize_plateau/optimize_deflated over
  M: MetricVocabulary + Clone (SweepPoint<M>: Clone bounds on M: Clone);
  the private vocabulary deleted; check_r_metric/generalization stay
  monomorphic on the RunMetrics vocabulary (the C10 wall by type);
  RegistryError::UnknownMetric carries the vocabulary's roster so the
  refusal prose derives instead of being hand-strung (byte-identical for R).

Bit-identity (C18): rng consumption order, centring, Laplace arithmetic,
floors, tie rules, and refusal prose are all unchanged for RunMetrics
inputs — the full registry suite (75 tests) passes unchanged except three
syntax-only edits for the UnknownMetric variant reshape; clippy -D warnings
clean on the three touched crates. Alternatives considered: a vocabulary
table in aura-core (rejected: inverts C28's supplied-from-outer-rungs
direction and cannot host the null model); a parallel measurement-side
deflation path (rejected: duplicates the machinery and adds a fifth
roster). Part 2 lands the consumers (campaign roster single-sourcing, the
IC implementor, acceptance tests).

refs #147

crates/aura-engine/src/lib.rs

@@ -79,7 +79,7 @@ pub use sweep::{
 // `resample_block`/`MetricStats` are foundation-grade (aura-analysis, #291);
 // re-exported here (unchanged exported name set) so existing
 // `aura_engine::{resample_block, MetricStats}` consumers stay source-compatible.
-pub use aura_analysis::{resample_block, MetricStats};
+pub use aura_analysis::{one_sided_p_laplace, resample_block, MetricStats, MetricVocabulary};
 pub use walkforward::{
     param_stability, walk_forward, RollMode, WalkForwardError, WalkForwardResult,
     WindowBounds, WindowOutcome, WindowRoller, WindowRun,