refactor(ml): extract PPO module into ml-ppo crate (task 7)

Move 24 PPO source files + flow_policy/ from ml into standalone
ml-ppo crate. The ml crate's ppo module is now a thin re-export layer
(`pub use ml_ppo::*`) plus two bridge files (trainable_adapter.rs,
stress_testing.rs) that depend on ml-internal types.

Key changes:
- ml-ppo: 25 modules (incl flow_policy subdir), 198 tests, standalone
- ml: depends on ml-ppo, re-exports via ppo/mod.rs
- Import rewrites: crate::common::action → ml_core::action_space,
  crate::dqn::{mixed_precision,xavier_init} → ml_core::*,
  crate::gradient_accumulation → ml_core::gradient_accumulation
- ml tests: 1929 pass (down from 2127 — 198 moved to ml-ppo)
- Workspace: 0 errors

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

crates/ml/Cargo.toml

@@ -70,6 +70,7 @@ colored = "2.1"  # Terminal color output for evaluation reports
 # Internal workspace crates
 ml-core.workspace = true
 ml-dqn.workspace = true
+ml-ppo.workspace = true
 config.workspace = true
 common = { workspace = true, features = ["questdb"] }
 risk = { path = "../risk" }