Add the streaming analogue of invoke() returning BoxStream<ResponseEnvelope>.
Security invariants are identical to invoke() (internal: false,
forwarded_for: None, same capabilities/scoped_env/ACL) — shared via a
build_root_context_inner helper with a bounded flag. The streaming path
sets deadline: None (unbounded subscriptions, ADR-049 §6). Calls
OperationRegistry::invoke_streaming() (already on develop). to_mcp is
unchanged (MCP excludes Subscription, ADR-041).
Tests cover: subscription dispatch, leading-slash strip, unknown op
NOT_FOUND, internal op NOT_FOUND (not leaked), None identity FORBIDDEN,
Query op INVALID_OPERATION_TYPE, invoke() on Subscription returns
INVALID_OPERATION_TYPE (guard holds through gateway), and
build_root_context_streaming sets deadline: None while carrying the
registration bundle.