SDK 10: implement AdCP 3.3 reporting subscriptions in the reliable-reporting runtime #1695

Python parity for adcontextprotocol/adcp-client#3504, on next (SDK 10 / AdCP 3.3).

Why

SDK 10 on AdCP 3.3.0-beta.1 ships the generated types for principal-level reporting subscriptions (RFC adcontextprotocol/adcp#8543 Part B, adcontextprotocol/adcp#8552), but no runtime. A seller built on the SDK can't list sync_principal in configuration_tasks, and a buyer has no helper to calculate the periods it's owed. Obligations, the ledger and receipts already live in the SDK, so the subscription runtime belongs here too. A host supplies only what it alone knows: which accounts a principal may read, and which buys an account holds.

Scope

The requirements are those of adcontextprotocol/adcp-client#3504, mapped to the Python runtime:

1. Principal runtime. Opt-in acceptance, validation, readback (core/reporting-subscription-state.json) and versioning of sync_principal reporting_subscriptions, plus principal.changed with reason reporting_subscription_changed. With the option off, keep today's behaviour.
2. Obligations. Host hooks for authorized accounts, authorization changes and buy facts. Selector resolution (exact / finest / all, ties to the lowest offering_id), precedence (a caller's own sync_accounts config beats any subscription; account_ids beats all-accounts), cutovers at period boundaries, account timezones, no clipping, and deterministic max_accounts.
3. Status and lost authority. subscription_coverage[] with exceptions only, each over [from, until); accounts_unauthorized[]; receipts accepted after loss for status_retention_days; re-grant, with and without backfill_on_regrant.
4. Managed delivery. Destinations from reporting_destinations, path-template expansion, account_id on every row, and per-account destinations.
5. Capabilities. configuration_tasks, configuration_task only when sync_accounts is listed, the subscriptions block, and media_buy.reporting_subscriptions.
6. Storage. adcp-reporting-storage PostgreSQL migrations for subscription versions, resolutions, exceptions and authority intervals; history retained as long as the obligations that cite it.
7. Buyer. A pure expected_subscription_periods(...) coverage function over plain data, and consumer-runtime and client support.
8. Conformance. Buyer-computed periods must equal seller-owed periods, period by period, across the cases listed in adcp-client#3504.

The spec is normative where it differs from the issue text. In particular, coverage starts at the first boundary at or after the latest of the version's accepted_at, the buy's confirmed_at and the account's authorization.authorized_at (see .changeset/pre/reporting-subscriptions.md in the spec repo).

10.0.0-beta.2 waits for this and for #1692.