feat(v3-ref-seller): persist packages upstream + implement update_media_buy

create_media_buy now POSTs each requested package as an upstream line
item (POST /v1/orders/{id}/lineitems) and returns the line_item_id as
the AdCP package_id. update_media_buy is no longer UNSUPPORTED_FEATURE:
cancel / pause / creative-assignment patches are applied to a
seller-local shadow store (self._buy_state), and creative_assignments
flow upstream via POST .../lineitems/{li}/creative-attach. get_media_buys
projects each line item plus its shadow-store entry so list-targeting,
measurement_terms, and per-package cancel/pause survive create → get.
Per-buy double-cancel raises NOT_CANCELLABLE.

Unblocks the storyboard residuals tracked in #706: pending_creatives_to_start,
inventory_list_targeting, invalid_transitions, and creative_fate_after_cancellation.

Author: bokelley