audit(0075): cycle close — drift-clean; doc-link + trade_rs tripwire tidy; drop ephemera

Architect drift review (c99e7b8..2e77ab1) against the design ledger +
CLAUDE.md invariants: drift-clean, debt only — no contract or spec
violation. Verified intact: C18 wire shape (trade_rs is #[serde(skip)] +
excluded from the hand-written PartialEq; runs.jsonl/families.jsonl bytes
and digests unmoved; mc_r_bootstrap is a printed line, never a record),
C1 determinism (seeded r_bootstrap + disjoint walkforward windows, both
pinned by byte-identical-rerun e2e tests), C7 frictionless Stage-1
(round_trip_cost stays 0.0), C8/C12 streaming reduction (IS sweep folds
O(trades)/member, OOS run non-reduce). The verbatim summarize_r /
r_metrics_from_rs float duplication is guarded by the cross-reducer
equality test — sound.

Tidy applied this close:
- doc-link drift: the new E[R] notation in r_bootstrap / RBootstrap /
  run_mc_r_bootstrap doc-comments was parsed by rustdoc as an intra-doc
  link `[R]` (4 warnings). Wrapped in backticks; doc build now clean.
- coverage micro-gap [low, architect]: added a tripwire asserting a
  populated trade_rs is absent from the serialized JSON (the serde(skip)
  C18-preservation property had no direct test — only Vec::new() was
  serialized before).

Carry-on (benign debt, not fixed): the mc usage literal carries a `usage:`
prefix the sibling sweep/walkforward usages omit — required because the
new `["mc", rest @ ..]` arm now handles the `--real` rejection and the
preserved golden asserts stderr.contains("usage"); churning the golden to
drop the prefix is not worth it.

Removed the cycle's ephemeral spec + plans (docs/specs/0075,
docs/plans/0075, docs/plans/0076) per the lifecycle rule.

Verified: cargo test --workspace 610 passed / 0 failed, clippy -D warnings
clean, cargo doc --no-deps clean.

closes #139

crates/aura-engine/src/mc.rs

@@ -153,7 +153,7 @@ where
     McFamily { draws, aggregate }
 }
 
-/// Distribution of E[R] under a moving-block bootstrap of an OOS per-trade R series
+/// Distribution of `E[R]` under a moving-block bootstrap of an OOS per-trade R series
 /// (#139). `block_len == 1` is the i.i.d. trade-shuffle; `block_len > 1` resamples
 /// contiguous runs, preserving the serial correlation of sequential trades a pure
 /// shuffle would erase. Deterministic (C1) given `seed`.

crates/aura-engine/src/report.rs

@@ -1477,6 +1477,16 @@ mod tests {
         assert_eq!(a, b);
     }
 
+    #[test]
+    fn populated_trade_rs_is_absent_from_serialized_json() {
+        // the in-memory conduit must never reach the wire: a populated trade_rs is
+        // dropped by #[serde(skip)], so the C18 on-disk shape is byte-unperturbed (#139).
+        let m = summarize_r(&pm_record_two_closed_trades(), 0.0);
+        assert_eq!(m.trade_rs, vec![2.0, -1.0], "precondition: trade_rs is populated");
+        let json = serde_json::to_string(&m).expect("RMetrics serializes");
+        assert!(!json.contains("trade_rs"), "trade_rs must not reach the wire: {json}");
+    }
+
     /// A minimal dense PositionManagement record with two closed trades at R = +2, -1.
     /// Columns per `r_col` (CLOSED=0, REALIZED_R=1, DIRECTION=4, ENTRY_PRICE=6,
     /// STOP_PRICE=7, CONVICTION_AT_ENTRY=9, SIZE=10, OPEN=11, UNREALIZED_R=12); width 13.