fix(sp21): Return v3.1 — drop short-rollout guard for volume bars (atomic)

Single-file fix to `compute_epoch_financials`: remove the
`if n_returns_f >= bars_per_year` short-rollout fallback that
left v2 semantics in place for sub-year rollouts. For Foxhunt's
volume bars, `bars_per_day ≈ 34_496` → `bars_per_year ≈ 8.69M`,
while a training epoch produces `n_returns ≈ 4.10M`. The guard
fired on every production epoch, so the v3 CAGR fix was a no-op.

Diagnosis chain:
- v3 commit (2937da889) merged the n_returns >= bars_per_year guard
- Smoke v4 (commit 62b5a50e8, workflow train-frv8x) epoch 1 showed
  Return=+2.963e2% — bit-identical to v1's pre-fix output
- Hypothesis 1 (cache poisoning): ruled out — ensure-binary log
  shows "Cache MISS: compiling binaries for 62b5a50e8" and ml crate
  was recompiled fresh
- Hypothesis 2 (different commit): ruled out — workflow params confirm
  commit-sha = 62b5a50e8 = current HEAD
- Hypothesis 3 (bars_per_year mismatch): confirmed — v4 log emits
  "Bars per day (from data): 34496" which makes bars_per_year > n_returns
  and triggers the v2 fallback inside the v3 branch

Fix: unconditional CAGR. The log-space clamp [-23, +20] bounds
the display in all edge cases (tests with tiny n_returns
extrapolate aggressively; the clamp caps at exp(20) - 1 ≈ +4.85e8%).

Expected v5 epoch 1 Return: ~+1.770e3% (was +2.963e2% under v4).
The new value is the *actual annualized* projection: 1377% over
a 0.47-year rollout. Overfit cycles cap at +4.85e8% (was +e19%).

Tests:
- cargo test -p ml --lib financials → 7/7
- Sign-only assertions in test cases (all > 0.0) — no regressions

Files changed:
- crates/ml/src/trainers/dqn/financials.rs: 1 conditional removed,
  comment block updated with v3 → v3.1 history
- docs/dqn-wire-up-audit.md: diagnosis + fix entry for 2026-05-11

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

crates/ml/src/trainers/dqn/financials.rs

@@ -80,13 +80,15 @@ pub(crate) fn compute_epoch_financials(
     // bars): annualized = exp(45.93 × 19_656 / 4_096_000) − 1 ≈ 24.7% —
     // a reasonable, interpretable number.
     //
-    // History (do not regress to either earlier form):
+    // History (do not regress to any earlier form):
     //   v1 (pre-2026-04): mean_per_bar × bars_per_year (arithmetic
     //       annualization) — produced -51,234% on negative per-bar means.
     //   v2 (2026-04..2026-05-11): log_growth.exp() − 1 (total compounded,
     //       no annualization) — produced +e19% on long rollouts.
-    //   v3 (this commit, 2026-05-11): annualized compounded — bounded by
-    //       construction and operationally meaningful.
+    //   v3 (2026-05-11): annualized compounded for n_returns >= bars_per_year,
+    //       fallback to v2 below it — but the fallback subsumed all production
+    //       runs on volume bars (bars_per_year ≈ 8.7M > n_returns ≈ 4.1M).
+    //   v3.1 (2026-05-11): unconditional CAGR — the clamp handles edge cases.
     //
     // Lower bound: -100% (log_growth.exp() − 1 ≥ -1 when log_growth ≥
     // log(1e-10) ≈ -23 per the per-bar clamp). Upper bound: practically
@@ -112,20 +114,25 @@ pub(crate) fn compute_epoch_financials(
         // annualization factor exceeds 1, which would extrapolate a
         // brief positive run to absurd annualized magnitudes (a 5-bar
         // test with 1.5% gain would annualize to e8% on 1-min bars).
-        // For sub-year rollouts, report TOTAL compounded growth
-        // instead — same `log_growth.exp() − 1` as the v2 formula,
-        // bounded by the per-bar floor to ±100% per the inflation
-        // audit's WR=46% honest meter discipline.
+        // **v3.1 (2026-05-11)**: dropped the `n_returns >= bars_per_year`
+        // short-rollout guard. The guard fell back to raw `log_growth`
+        // (= v2 formula) whenever the rollout was shorter than a trading
+        // year. For Foxhunt's volume bars the data extracts
+        // `bars_per_day ≈ 34_496` → `bars_per_year ≈ 8.7M`, but a single
+        // training epoch produces `n_returns ≈ 4.1M` step returns. Result:
+        // every production epoch took the v2 branch and the v3 fix was a
+        // no-op — smoke v4 epoch 1 (commit 62b5a50e8) reproduced v1's
+        // `Return=+2.963e2%` exactly.
+        //
+        // Unconditional CAGR is the correct invariant: "what annualized
+        // return is this policy producing?" works for any rollout length.
+        // Tests (very small n_returns) get extrapolated (a 5-bar +1.5%
+        // test extrapolates to e8% nominally) — the log-space clamp
+        // `[-23, +20]` bounds the display magnitude, and the test cases
+        // below assert ordering / sign, not exact magnitudes.
         let n_returns_f = n_returns as f64;
         if log_growth.is_finite() && n_returns_f > 0.0 && bars_per_year > 0.0 {
-            let log_scaled = if n_returns_f >= bars_per_year {
-                // Production rollout (typically ~4M bars vs ~20K bars/year)
-                // — annualize for CAGR semantics.
-                log_growth * bars_per_year / n_returns_f
-            } else {
-                // Short rollout (tests, warmup) — report total compounded.
-                log_growth
-            };
+            let log_scaled = log_growth * bars_per_year / n_returns_f;
             // Sanity bounds: lower at -23 (≈ -100% by construction
             // from per-bar `(1+r).max(1e-10)` floor), upper at +20
             // (≈ exp(20) − 1 ≈ +4.85e8% — beyond this magnitude the