Add the foundational types for ADR-049 streaming handlers:
- StreamingHandler, ResponseStream type aliases and HandlerKind enum
(Once | Stream) in registration.rs, with make_streaming_handler() helper
- CallError::invalid_operation_type() in wire.rs (sixth protocol code,
retryable: false)
- HandlerRegistration.handler flipped from Handler to HandlerKind;
HandlerRegistration::new() now takes HandlerKind
- OperationRegistryBuilder absorbs wrapping: with_local/with_leaf/
with_leaf_provenance wrap raw Handler in HandlerKind::Once for
Query/Mutation; new with_local_streaming/with_leaf_streaming take a
StreamingHandler and wrap in HandlerKind::Stream for Subscription.
Builder validates kind matches spec.op_type (mismatch = startup error)
- OperationRegistry::register() returns Result<(), String> with a clear
mismatch message; all call sites updated to handle the Result
- invoke() matches on HandlerKind: Once -> existing path; Stream ->
INVALID_OPERATION_TYPE error envelope (guards against silent
truncation; invoke_streaming() added in a downstream task)
- OverlayOperationEnv::invoke_with_policy matches on HandlerKind:
Once -> dispatch; Stream -> INVALID_OPERATION_TYPE (composition is
request/response-only)
- Migrated every HandlerRegistration::new() construction site (~95)
to wrap raw Handler in HandlerKind::Once(handler); the builder sites
are handled by the builder-absorbs-wrapping change
- Updated two websocket subscription tests that relied on Subscription
ops dispatching via invoke() to expect INVALID_OPERATION_TYPE
- Added unit tests for invoke/register validation and
make_streaming_handler
