feat(0075): walk-forward strategy-selectable + R-reporting (iter 1)

Make `aura walkforward --strategy stage1-r [--real <SYM>]` roll IS->OOS
windows, sweep the stage1-r grid in-sample, pick the winner by an R metric
(sqn_normalized), run it out-of-sample, and report per-window + pooled OOS
R-metrics. The bare SMA walkforward path is byte-identical (its dispatch arm
is verbatim today's body; the pooled `oos_r` block is emitted only when a
window carries an `r` block).

Engine:
- RMetrics gains an in-memory `trade_rs: Vec<f64>` (realised R per closed
  trade), excluded from serde (`#[serde(skip)]`) and from a hand-written
  PartialEq, so the C18 wire shape and every existing equality assertion /
  round-trip stay unchanged. summarize_r now retains the per-trade R vector
  it used to drop.
- New `r_metrics_from_rs(&[f64])` reduces a flat (pooled across-window) R
  series to RMetrics. Its R-distribution arithmetic is copied verbatim from
  summarize_r (the byte-pinned floats must not be algebraically refactored);
  the two copies are guarded in lockstep by a cross-reducer equality test.
  net_expectancy_r = expectancy_r (exact at the Stage-1 cost=0 invariant);
  conviction_terciles_r = [0,0,0] (per-trade conviction is not pooled).

CLI:
- walkforward gains --strategy + the four stage1-r grid flags (reusing
  parse_csv_list / Stage1RGrid); walkforward_family is strategy-dispatched.
- Windowed stage1-r helpers: stage1_r_sweep_over (reduce-mode folded IS
  sweep, O(trades)/member) and run_oos_r (non-reduce OOS run, for the
  stitched pip-equity curve), plus stage1_r_space.

Frictionless Stage-1 R (costs are Stage-2). Verified: cargo build clean,
full `cargo test --workspace` green (SMA walkforward, synthetic mc,
stage1_r_single_run_output_golden, and the C18 round-trips all preserved),
clippy -D warnings clean. Monte-Carlo R-bootstrap is iter 2.

refs #139

crates/aura-engine/src/lib.rs

@@ -62,8 +62,9 @@ pub use harness::{
     VecSource,
 };
 pub use report::{
-    derive_position_events, f64_field, join_on_ts, summarize, summarize_r, ColumnarTrace,
-    JoinedRow, PositionAction, PositionEvent, RMetrics, RunManifest, RunMetrics, RunReport,
+    derive_position_events, f64_field, join_on_ts, r_metrics_from_rs, summarize, summarize_r,
+    ColumnarTrace, JoinedRow, PositionAction, PositionEvent, RMetrics, RunManifest, RunMetrics,
+    RunReport,
 };
 pub use sweep::{
     sweep, GridSpace, ParamRange, RandomSpace, Space, SweepError, SweepFamily, SweepPoint,