v0.9 ticket 01. Independent exploration_sessions/exploration_messages
entities (never merged into EvalRun, keeping ADR-0001/0002 semantics
intact): create/message/close APIs forward virtual-user messages through
the target's real channel, persist both parties' rows with latency, and
close with a whitelist-normalized experience record. Budget enforcement
is a platform ledger — sessions per window, turns per session, and
session interval overruns return 409 with readable reasons; accelerated
lines accept manual sessions only. Messages delivered but unanswered
still consume a turn so timeouts cannot bypass the budget.
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