arch(crt-a): delete decision_stride field — greenfields atomic refactor

Per spec 2026-05-20-continuous-reasoning-trader-design.md §3.3 and §8:
decision_stride is REMOVED, not deprecated. No backwards-compat shim,
no fallback. Every consumer migrates in this commit per
feedback_no_partial_refactor.

Removed from:
  - bin/fxt-backtest: RunArgs CLI flag, SweepBase field, SweepCell
    override field, default_decision_stride() function, all three
    BacktestHarnessConfig and RunArgs construction sites
  - crates/ml-backtesting/src/harness.rs: BacktestHarnessConfig field,
    MultiHorizonLoaderConfig decision_stride initializer, `let stride`
    local, `if event_count % stride == 0` gate around
    step_decision_with_latency; forward_step_into + step_decision now
    share a single window-full guard (merged into one `if` block)
  - crates/ml-alpha/src/data/loader.rs: MultiHorizonLoaderConfig field,
    next_sequence stride logic simplified to stride=1 (consecutive
    snapshots only)
  - crates/ml-alpha/src/trainer/perception.rs: PerceptionTrainerConfig
    field and Default impl; all four dt_s locals replaced with 1.0_f32
    (training K-loop, graph-capture K-loop, forward_step_into CfC step,
    eval K-loop)
  - crates/ml-alpha/examples/alpha_train.rs: CLI flag, trainer_cfg and
    both loader configs
  - crates/ml/examples/alpha_baseline.rs: CLI flag, train + eval stride
    gates replaced with unconditional read_all()
  - config/ml/*.yaml: decision_stride: lines removed from
    sweep_smoke, sweep_threshold_tuning, sweep_deployability,
    sweep_decision_stride_example (file repurposed as generic example)
  - tests: forward_step_golden, perception_overfit (×7 structs including
    the stride=4 smoke repurposed as a second convergence check),
    multi_horizon_loader (stride=4 spacing test repurposed as
    ts_ns monotonicity check), ring3_replay, trainer_parity

Harness loop now invokes BOTH forward_step_into AND
step_decision_with_latency on every event whenever the snapshot window
is full. forward_step_into advances SSM state and writes alpha_probs_d;
step_decision_with_latency reads alpha_probs_d immediately after —
no CPU roundtrip, no stride gate.

n_decisions ≈ events_processed - seq_len + 1 after this commit
(vs ~9999 at stride=200 in the S2 baseline).

cargo check --workspace: clean
cargo test -p ml-backtesting --lib: 33 passed
cargo test -p ml-alpha --lib: 33 passed (6 ignored)

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

crates/ml/examples/alpha_baseline.rs

@@ -226,14 +226,6 @@ struct Cli {
     #[arg(long, default_value_t = 0.99)]
     gamma: f32,
 
-    /// Decision stride — emit a new action only every N steps; between
-    /// decisions force `action=0` (wait), preserving the previously-opened
-    /// position. Aligns DQN decision cadence with the multi-minute alpha
-    /// horizon and stops the policy from flip-flopping on per-bar noise
-    /// ("coin-flip problem"). 1 = current per-bar behaviour. Pair with
-    /// γ→0.999+ so the Bellman chain reaches across the wait segments.
-    #[arg(long, default_value_t = 1)]
-    decision_stride: u32,
     #[arg(long, default_value_t = 0.9)]
     alpha_m: f32,
     #[arg(long, default_value_t = 0.03)]
@@ -660,16 +652,8 @@ fn main() -> Result<()> {
                 }
             }
             stream.synchronize()?;
-            // `--decision-stride N`: only emit a new action every N steps;
-            // between strides force action=0 (wait) so an open position is
-            // held rather than re-decided per bar. Avoids the per-bar
-            // "coin-flip" overtrading. step==0 is always a decision.
-            let stride_train = cli.decision_stride.max(1) as usize;
-            let actions = if step % stride_train == 0 {
-                batched_action_pinned_train.read_all()
-            } else {
-                vec![0_i32; train_n_par]
-            };
+            // A1: decision_stride removed; act on every step.
+            let actions = batched_action_pinned_train.read_all();
 
             for i in 0..train_n_par {
                 if done_flags[i] { continue; }
@@ -1011,17 +995,8 @@ fn main() -> Result<()> {
             }
             // ONE sync per step (instead of N).
             stream.synchronize()?;
-            // `--decision-stride N`: same rate-limit semantics as training.
-            // Between strides force action=0 so an open passive limit
-            // post / live position is held rather than re-decided. Cuts
-            // per-bar trade counts ~stride× and removes the coin-flip
-            // overtrading on noise.
-            let stride_eval = cli.decision_stride.max(1) as usize;
-            let actions = if step % stride_eval == 0 {
-                batched_action_pinned.read_all()
-            } else {
-                vec![0_i32; n_par]
-            };
+            // A1: decision_stride removed; act on every step.
+            let actions = batched_action_pinned.read_all();
             // Step all envs on CPU.
             for i in 0..n_par {
                 if done_flags[i] { continue; }