refactor: remove trading_engine dep from ml and risk crates

Re-export HardwareTimestamp through data crate instead of ml/risk
depending directly on trading_engine. Reduces coupling between
the ML pipeline and the trading engine.

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

crates/ml/Cargo.toml

@@ -68,7 +68,6 @@ reqwest.workspace = true
 colored = "2.1"  # Terminal color output for evaluation reports
 
 # Internal workspace crates
-trading_engine.workspace = true
 config.workspace = true
 common.workspace = true
 risk = { path = "../risk" }