refactor: cut the engine's backtest-metrics edge via RunReport<M>

C28 phase 2 (Stratification); realizes item 1 of the deferred #147. The
engine's production surface no longer names a backtest-metric type:

- RunReport becomes generic over its metric payload M; sweep/mc/walkforward/
  blueprint thread the parameter (SweepPoint<M>, SweepFamily<M>, WindowRun<M>,
  WalkForwardResult<M>). RunManifest stays concrete and engine-owned (its
  selection: Option<FamilySelection> embeds the foundation-grade analysis type).
- summarize and the MC assembly (McDraw/McFamily/McAggregate/RBootstrap/
  r_bootstrap/monte_carlo) move to aura-backtest - McAggregate::from_draws reads
  RunMetrics fields by name, so generifying it is the phase-6 metric-vocabulary
  abstraction (#147 item 2), still deferred; wholesale relocation is the honest
  cut. The concrete instantiation lives in aura-backtest as
  `type RunReport = aura_engine::RunReport<RunMetrics>` + sibling aliases.
- the statistics kernel (MetricStats/quantile/resample_block/SplitMix64) moves
  to the aura-analysis foundation; the engine re-imports it (inner->foundation,
  legal) and re-exports it so existing consumers stay source-compatible.

Dependency inversion in one commit: aura-engine drops aura-backtest from
[dependencies] (back to dev-deps for its SimBroker/RunMetrics test fixtures);
aura-backtest gains aura-engine. Cycle-free for lib targets - the cycle closes
only through the engine's dev-dep edge, the pattern aura-vocabulary already
uses. aura-backtest reaches the kernel transitively through the engine
re-export, so no aura-backtest -> aura-analysis edge exists (the C28 ladder
permits backtest -> {core, engine} only). run_indexed / SplitMix64::next_f64
widened pub(crate) -> pub for cross-crate use.

Consumers (registry/campaign/cli/composites/ingest/bench) rewired by import
path only, no call-site logic changed. The c28_layering structural test extends
to the full ladder: aura-analysis (no aura-* deps), aura-engine ⊆ {core,
analysis}, aura-backtest ⊆ {core, engine}.

Behaviour-preserving: 1448/0 tests, clippy -D warnings clean, serde shapes
byte-identical (C18 - RunReport<M> keeps field order manifest,metrics; the
CLI pre-serialized-splice contract unchanged), moved code traceable via git
rename detection. Cycle-introduced broken intra-doc links fixed.

closes #292

crates/aura-cli/src/campaign_run.rs

@@ -19,11 +19,12 @@ use std::num::NonZeroUsize;
 use std::path::{Path, PathBuf};
 use std::sync::{mpsc, Arc};
 
+use aura_backtest::{summarize, summarize_r, RunReport};
 use aura_campaign::{CampaignOutcome, CellOutcome, CellSpec, ExecFault, MemberFault, MemberRunner};
 use aura_core::{Cell, ParamSpec, PrimitiveBuilder, Scalar, ScalarKind, Timestamp};
 use aura_engine::{
-    blueprint_from_json, f64_field, summarize, summarize_r, ColumnarTrace, Composite,
-    FamilySelection, RunReport,
+    blueprint_from_json, f64_field, ColumnarTrace, Composite,
+    FamilySelection,
 };
 use aura_ingest::{instrument_geometry, open_columns_window, unix_ms_to_epoch_ns};
 use aura_registry::{CampaignRunRecord, WriteKind};

crates/aura-cli/src/main.rs

@@ -16,13 +16,13 @@ use render::{ChartData, ChartMeta, ChartMode, ReduceKind, Series};
 use aura_core::{zip_params, Cell, Firing, ParamSpec, PrimitiveBuilder, Scalar, ScalarKind, Timestamp};
 use aura_composites::{cost_graph, risk_executor, StopRule};
 use aura_engine::{
-    blueprint_from_json, blueprint_to_json, f64_field, join_on_ts, monte_carlo, param_stability,
-    r_metrics_from_rs, summarize, summarize_r, walk_forward, window_of, BindError, BlueprintNode,
-    Composite, FamilySelection, GraphBuilder, Harness, JoinedRow, McAggregate,
-    McFamily, MeasurementReport, RBootstrap, RollMode, RunManifest, RunMetrics, RunReport,
+    blueprint_from_json, blueprint_to_json, f64_field, join_on_ts, param_stability,
+    walk_forward, window_of, BindError, BlueprintNode,
+    Composite, FamilySelection, GraphBuilder, Harness, JoinedRow,
+    MeasurementReport, RollMode, RunManifest,
     SelectionMode,
-    SweepFamily, SweepPoint, SyntheticSpec, VecSource, WalkForwardResult,
-    WindowBounds, WindowRoller, WindowRun,
+    SyntheticSpec, VecSource,
+    WindowBounds, WindowRoller,
 };
 use aura_registry::{
     check_r_metric, group_families, mc_member_reports, optimize_deflated,
@@ -30,7 +30,11 @@ use aura_registry::{
     FamilyMember, Generalization, NameKind, PlateauMode, Registry, RunTraces,
     DEFLATION_BLOCK_LEN, DEFLATION_N_RESAMPLES,
 };
-use aura_backtest::{SimBroker, PM_FIELD_NAMES, PM_RECORD_KINDS};
+use aura_backtest::{
+    monte_carlo, r_metrics_from_rs, summarize, summarize_r, McAggregate, McFamily, RBootstrap,
+    RunMetrics, RunReport, SimBroker, SweepFamily, SweepPoint, WalkForwardResult, WindowRun,
+    PM_FIELD_NAMES, PM_RECORD_KINDS,
+};
 use aura_std::{
     GatedRecorder, LinComb, Recorder, RollingMax, RollingMin, SeriesReducer, Sub,
 };
@@ -6347,7 +6351,7 @@ mod tests {
         // breaks a test instead of silently shipping. Pins the user-visible wire
         // shape of the mc R-bootstrap render (the parser + engine primitive are
         // covered elsewhere; this is the output-shape layer).
-        let boot = aura_engine::r_bootstrap(&[1.0, -0.5, 2.0, -1.0], 64, 2, 7);
+        let boot = aura_backtest::r_bootstrap(&[1.0, -0.5, 2.0, -1.0], 64, 2, 7);
         let line = mc_r_bootstrap_json(&boot);
         let v: serde_json::Value = serde_json::from_str(&line).expect("canonical json line");
         let obj = &v["mc_r_bootstrap"];

crates/aura-cli/src/verb_sugar.rs

@@ -363,7 +363,7 @@ pub(crate) fn run_sweep_sugar(
 /// itself enforces.
 fn cross_instrument_members(
     outcome: &aura_campaign::CampaignOutcome,
-) -> Vec<aura_engine::RunReport> {
+) -> Vec<aura_backtest::RunReport> {
     outcome
         .cells
         .iter()
@@ -1180,7 +1180,7 @@ mod tests {
     /// A minimal `RunReport` stamped with `instrument`, distinguishable in a
     /// members list by nothing but that stamp — the field
     /// `cross_instrument_members` preserves verbatim from a nominating cell.
-    fn fake_report(instrument: &str) -> aura_engine::RunReport {
+    fn fake_report(instrument: &str) -> aura_backtest::RunReport {
         aura_engine::RunReport {
             manifest: aura_engine::RunManifest {
                 commit: String::new(),
@@ -1194,7 +1194,7 @@ mod tests {
                 topology_hash: None,
                 project: None,
             },
-            metrics: aura_engine::RunMetrics {
+            metrics: aura_backtest::RunMetrics {
                 total_pips: 0.0,
                 max_drawdown: 0.0,
                 bias_sign_flips: 0,