Cell 05 — Mock vendor APIs. Five pure-Python vendor modules (airline, cab, restaurant, hotel, payment) consolidated into one cell. Each exposes a frozen `*State` dataclass plus five helpers (`dispatch`, `initial_state`, `apply_schema_mutation`, `describe_schema`, `emit_side_channel_if_pending`) and a `TOOLS` registry. Implements `docs/modules/vendors.md` §§2–8: three schema versions per domain, integer-INR monetary invariant, deterministic timeout via `hash((seed,tool,args)) & 0x7F == 0`, per-domain idempotency keys returning `DUPLICATE_*` policy errors, consumed-on-read side-channel notices, and cross-domain auth cascades from `payment.charge`.