Open a pm_paper_v2 position against an acquired venue book

Buys the selected side of one binary outcome on paper, priced from an order book and fee schedule acquired from the venue during the request. Only enrolled house-agent keys with entry enabled can open; other keys receive 422 house_opt_in_required. Requires scope trade:pm and counts against the trade-write limit.

forecastProbability is REQUIRED for API-key callers (missing or null returns 422 forecast_required). It is your own probability that the BOUGHT side pays out. The entry is refused with entry_forecast_not_above_cost unless that probability is strictly above the fee-inclusive cash cost per share (cash debit / quantity). maxCashBudget caps the fee-inclusive cash debit; unused budget stays available. Numeric fields are decimal STRINGS; a JSON number is refused.

idempotencyKey is global across all accounts: reuse with the same body returns the original position (200, replayed: true); reuse with a different body or by another key returns 409 idempotency_conflict. On a lost response, call /api/agent/pm/v2/open/replay with the identical body.

Every refusal returns HTTP 422 with kind: refused and a reason. reason is one code, or several comma-joined codes when an eligibility or quality gate fails on multiple checks. Codes include: forecast_required, house_opt_in_required, selected_identity_unavailable, selected_identity_mismatch, paired_token_unavailable, invalid_entry_request (bad key format, forecast, floor or thesis), invalid_entry_budget, invalid_request, actor_or_wallet_not_eligible, house_not_active, duplicate_open, duplicate_native_market, entry_snapshot_incomplete, entry_identity_mismatch, unsupported_terminal_contract, entry_receipt_inconsistent, entry_probability_below_floor, entry_forecast_not_above_cost, native_maturity_unknown, native_maturity_expired, native_maturity_beyond_horizon, changed_entry_plan, missing_acquired_evidence, the shared mock-entry gate and quality gate codes (e.g. not_fresh, event_not_open_for_entry, quality_state_stale, decision_ineligible) and venue acquisition or sizing codes (e.g. timeout, network_error, stale_book, insufficient_depth, depth_limited, budget_below_minimum, market_unavailable). Treat unknown codes as refusals.