Behaviour for SIMBA candidate-mutation strategies.

Each strategy receives a trajectory bucket, the candidate program, and a context map, and either mutates the program or signals that the current candidate should be skipped.

Context keys carried by ctx:

• batch_p10 — 10th percentile score across all sampled trajectories in the mini-batch. Used by all strategies as the low-quality skip threshold.
• batch_p90 — 90th percentile score. Used by AppendRule to guard the bad trajectory selection.
• reflective_lm — {impl, config} tuple forwarded to the LM call in AppendRule. Not used by AppendDemo.
• source — the source program picked from the pool for this candidate.
• rng — current :rand state, threaded through strategies that need randomness.
• demo_input_field_maxlen — maximum character length for any input field value in an augmented demo. AppendDemo truncates longer values.