feat(nisps/ml): crystallise Explore-and-place into shared FeedbackController
Add FeedbackMode::ExploreAndPlace + Idle/Exploring/Placing state machine
(no-heap, deterministic nisps::Rng): enter/exit_explore, reroll, nudge, undo,
begin_place, commit_place, cancel_place + on_down/on_up browser policy. Wire
the nisps_ml_feedback_* C API + EXPORTED_FUNCTIONS, extend parity Stage 5d.
Fix set_mode(3) falling back to Avoid. Native ctest 4/4; parity native==WASM
within 1e-5 (max delta 2.4e-7). Rebuilt nisps.{js,wasm}.
22efb1c411