chore(ml-backtesting): migrate to MultiResolutionConfig

Backtest harness + tests use default_three_scale config (1:10,30:10,
100:12 = 1510 ticks of context). Replaces the previous seq_len=32
single-scale path which is now deleted.

crates/ml-backtesting/src/harness.rs

@@ -247,7 +247,7 @@ impl BacktestHarness {
         let loader_cfg = MultiHorizonLoaderConfig {
             files,
             predecoded_dir: cfg.predecoded_dir.clone(),
-            seq_len: 1,
+            multi_resolution: ml_alpha::data::aggregation::MultiResolutionConfig::default_three_scale(),
             horizons: ml_alpha::heads::HORIZONS,
             n_max_sequences: 0,
             seed: 0,

crates/ml-backtesting/tests/ring3_replay.rs

@@ -43,7 +43,7 @@ fn try_loader() -> Option<MultiHorizonLoader> {
     let cfg = MultiHorizonLoaderConfig {
         files,
         predecoded_dir: mbp10.clone(),
-        seq_len: 1,
+        multi_resolution: ml_alpha::data::aggregation::MultiResolutionConfig::default_three_scale(),
         horizons: ml_alpha::heads::HORIZONS,
         n_max_sequences: 0,
         seed: 0xCAFE_F00D,

crates/ml-backtesting/tests/trainer_parity.rs

@@ -30,7 +30,7 @@ fn try_loader(inference_only: bool) -> Option<MultiHorizonLoader> {
     let cfg = MultiHorizonLoaderConfig {
         files,
         predecoded_dir: mbp10.clone(),
-        seq_len: 1,
+        multi_resolution: ml_alpha::data::aggregation::MultiResolutionConfig::default_three_scale(),
         horizons: ml_alpha::heads::HORIZONS,
         n_max_sequences: 1,
         seed: 0xCAFEF00D,