feat(stage1-r): bias rename + stop-rule + position-management + summarize_r (iter 1)

Iteration 1 of the Stage-1 "R-based signal quality" cycle (spec 0065): a
strategy's signal quality, measured in R on its unsized bias stream, feed-forward.
New discrete-trade machinery, NOT a SimBroker extension.

What lands:
- exposure -> bias (refs #126): the Exposure node/type/file/output-field renamed to
  Bias (computation unchanged: clamp(signal/scale,-1,+1); the SEMANTICS change — the
  output is the unsized strategy bias, sizing leaves the strategy). Scoped to the
  semantic core; behaviour-preserving cosmetics are deferred (see below).
- stop-rule nodes (refs #119): VolStop = k*EMA(|price - prev_price|) (one fused node;
  the volatility that defines 1R, close-to-close — true-range ATR deferred, needs
  OHLC) + FixedStop (constant, the test fixture / fixed-vs-vol structural-axis sibling).
- PositionManagement (refs #127): the stateful heart. Latches the entry-cycle stop
  distance as the FROZEN R-denominator (never re-read), marks/exits no-look-ahead like
  SimBroker (a position earns from the next cycle; an exit realises against the cycle's
  close), and emits ONE dense per-cycle R-record (C8). Stop-outs are NOT capped at -1R
  (a gap through the stop realises R < -1 — the honest loss tail); R is computed
  size-invariantly (size = (exit-entry)*dir / latched_dist cancels). The trade ledger
  is the rows where closed_this_cycle; the R-equity is cum_realized + unrealized; a
  position open at window end is the last row (open=true) — explicit, never silent MtM.
- summarize_r + RMetrics (refs #129): the post-run fold (NOT an in-graph node —
  recorders drain post-run). E[R], win-rate, profit-factor, avg win/loss R, by-trade
  max R-drawdown, n_open_at_end (window-end force-close). SQN / conviction terciles /
  net-of-cost / RunMetrics.r are iteration 2.

Design adversarially hardened before implementation (12-juror refute panel; decisions
on #117). Ratified implementer deviations from the plan snippet, verified by hand:
- col-4 `direction` tracks the OPEN position at cycle end (window-end synthesis; names
  the reopened leg on a reversal), falling back to the closed trade's dir only when
  flat. summarize_r does not read col 4; the R-metrics are unaffected.
- .named("exposure") kept on the 4 previously-unnamed Bias nodes so the auto-derived
  param-path stays exposure.scale (no manifest/dir-name drift) — the unnamed nodes
  would otherwise flip to bias.scale.
- intra-doc links [`Exposure`] -> [`Bias`] in sim_broker/latch + the sample-model test
  pin (cosmetic, behaviour-neutral; broken intra-doc links fail cargo doc).
- the E2E drives a full bootstrapped Harness (stronger than the plan's direct-node
  drive — exercises the real cross-crate producer->consumer seam).

Deferred (behaviour-preserving, separate cosmetic pass — NOT this iteration):
RunMetrics.exposure_sign_flips / Metric::ExposureSignFlips, SimBroker/LongOnly
"exposure" input ports, the "exposure" tap/trace names, and the .named("exposure")
instance labels. Iteration 2: the Sizer seam, the RiskExecutor composite (+ the Veto
documented-seam), summarize_r enrichment, RunMetrics.r, and the CLI/recording surface.

Verification (orchestrator-run, not agent-claimed):
- cargo build --workspace: clean.
- cargo test --workspace: all green, 0 failed (incl. the new bias/stop_rule/
  position_management unit tests, the summarize_r arithmetic tests, and the
  stage1_r_e2e capstone + layout guard).
- cargo clippy --workspace --all-targets -- -D warnings: clean (exit 0).
- Keystone RED tests pass: no_lookahead_bias_exit_realises_the_held_move,
  stop_out_is_not_capped_at_minus_one_r, no_gap_stop_is_exactly_minus_one_r,
  reversal_closes_one_leg_and_reopens, open_at_window_end_is_carried_on_the_last_row.

refs #117 #119 #126 #127 #129

crates/aura-engine/src/builder.rs

@@ -204,7 +204,7 @@ mod tests {
     use crate::test_fixtures::sma_cross as hand_sma_cross;
     use crate::{Composite, OutField, Role, Target};
     use aura_core::{Firing, ScalarKind};
-    use aura_std::{Exposure, Recorder, SimBroker, Sma, Sub};
+    use aura_std::{Bias, Recorder, SimBroker, Sma, Sub};
     use std::sync::mpsc;
 
     /// The `sma_cross` sub-composite authored through the builder.
@@ -251,16 +251,16 @@ mod tests {
         let (tx_ex, _r2) = mpsc::channel();
         let mut g = GraphBuilder::new("root");
         let xross = g.add(built_sma_cross());
-        let expo = g.add(Exposure::builder());
+        let expo = g.add(Bias::builder());
         let broker = g.add(SimBroker::builder(0.0001));
         let eq = g.add(Recorder::builder(vec![ScalarKind::F64], Firing::Any, tx_eq));
         let ex = g.add(Recorder::builder(vec![ScalarKind::F64], Firing::Any, tx_ex));
         let src = g.source_role("src", ScalarKind::F64);
         g.feed(src, [xross.input("price"), broker.input("price")]);
         g.connect(xross.output("out"), expo.input("signal"));
-        g.connect(expo.output("exposure"), broker.input("exposure"));
+        g.connect(expo.output("bias"), broker.input("exposure"));
         g.connect(broker.output("equity"), eq.input("col[0]"));
-        g.connect(expo.output("exposure"), ex.input("col[0]"));
+        g.connect(expo.output("bias"), ex.input("col[0]"));
         let built_flat = g
             .build()
             .expect("root resolves")
@@ -280,12 +280,12 @@ mod tests {
         // wiring-totality check rejects it at compile (broker is node 1, slot 0).
         let (tx_eq, _r1) = mpsc::channel();
         let mut g = GraphBuilder::new("root");
-        let expo = g.add(Exposure::builder());
+        let expo = g.add(Bias::builder());
         let broker = g.add(SimBroker::builder(0.0001));
         let eq = g.add(Recorder::builder(vec![ScalarKind::F64], Firing::Any, tx_eq));
         let price = g.source_role("price", ScalarKind::F64);
         g.feed(price, [expo.input("signal"), broker.input("price")]);
-        // BUG: g.connect(expo.output("exposure"), broker.input("exposure")) missing.
+        // BUG: g.connect(expo.output("bias"), broker.input("exposure")) missing.
         g.connect(broker.output("equity"), eq.input("col[0]"));
         let compiled = g
             .build()
@@ -386,19 +386,19 @@ mod tests {
     fn simbroker_legs_resolve_by_name_and_a_typo_is_caught() {
         // exposure/price (both f64, slot 0/1) addressed by NAME — the #21 legibility win
         let mut ok = GraphBuilder::new("ok");
-        let expo = ok.add(Exposure::builder());
+        let expo = ok.add(Bias::builder());
         let broker = ok.add(SimBroker::builder(0.0001));
         let src = ok.source_role("p", ScalarKind::F64);
         ok.feed(src, [expo.input("signal"), broker.input("price")]);
-        ok.connect(expo.output("exposure"), broker.input("exposure"));
+        ok.connect(expo.output("bias"), broker.input("exposure"));
         ok.expose(broker.output("equity"), "eq");
         assert!(ok.build().is_ok());
 
         // a transposed/typo'd port name is caught, where a bare slot index is not
         let mut typo = GraphBuilder::new("typo");
-        let expo2 = typo.add(Exposure::builder());
+        let expo2 = typo.add(Bias::builder());
         let broker2 = typo.add(SimBroker::builder(0.0001));
-        typo.connect(expo2.output("exposure"), broker2.input("pirce"));
+        typo.connect(expo2.output("bias"), broker2.input("pirce"));
         assert_eq!(
             typo.build().err(),
             Some(BuildError::UnknownInPort { node: 1, name: "pirce".into() })