0076: C++ target-wide metadata reads

Status: accepted for development 0.9.

Decision

Provide metadata(ObjectRef) on C++ Production and ReadSession, matching the existing Rust, C and Python operation. Return copied std::vector<MetadataAssertion> values through Result; the native set is released before returning. Copies survive view/production disposal.

Retain assertion order, repeated properties and every typed value. Do not collapse assertions into a map or infer types from vocabulary definitions. The view overload reads its pinned knowledge; the production overload reads current knowledge. Existing bounded property queries remain appropriate for cross-target searches. Errors propagate through both established C++ styles, including builds without exceptions.

Impact

This closes a projection gap without changing C symbols/layouts, schema, protocol or package versions. Reviewed against the standards policy: no vocabulary mapping, normalization or domain meaning changes.