refactor(ml): extract observability, stress-testing, security into sub-crates

- ml-observability (1.2K lines): alerts, dashboards, metrics modules.
  Depends on ml-core + common (ModelType). 4 tests passing.

- ml-stress-testing (1.3K lines): load_generator, market_simulator,
  performance_analyzer modules. Depends on ml-core + common + config.
  5 tests passing.

- ml-security (1.4K lines): anomaly_detector, prediction_validator
  modules. Depends on ml-core + ml-ensemble (EnsembleDecision,
  ModelVote, TradingAction). 17 tests passing.

Total: 18 sub-crates extracted from ml monolith.
Workspace: 0 errors, ml tests 876 + 26 in new sub-crates.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

crates/ml/Cargo.toml

@@ -81,9 +81,12 @@ ml-checkpoint.workspace = true
 ml-data-validation.workspace = true
 ml-validation.workspace = true
 ml-risk.workspace = true
+ml-security.workspace = true
 ml-backtesting.workspace = true
 ml-asset-selection.workspace = true
 ml-universe.workspace = true
+ml-observability.workspace = true
+ml-stress-testing.workspace = true
 config.workspace = true
 common = { workspace = true, features = ["questdb"] }
 risk = { path = "../risk" }