Upgrade the seed explore/exploit gate from a flat reuse chance to a proper
bandit over the path-outcome arms:
- _v2_path_arm_beta: Beta posterior over an arm's liked-rate (liked>=0.3 success,
<0.0 failure, neutral ignored; Laplace prior)
- _v2_path_explore_decision: Thompson-sample theta per concept-matching arm vs a
fresh Beta(1,1) explore arm; good arms exploit, dislike-dominated arms route to
a fresh seed ("do something else")
- wired into _v2_choose_successful_seed (falls back to the flat gate with no data)
- shared _v2_path_matching_arms helper; _V2_KEY_REWARD + liked/disliked consts
5 tests incl. a statistical one: a strongly-liked arm rarely explores, a hated
arm almost always does.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
