- Make ServiceSwitcherStrategy inherit from BaseObject with properties
for services and active_service, and move initial service selection
into the base class
- Add on_service_switched event to ServiceSwitcherStrategy
- handle_frame now returns the switched-to service (or None), allowing
ServiceSwitcher to swallow ManuallySwitchServiceFrame on switch and
request metadata from the new active service
- Override push_frame to suppress RequestMetadataFrame and
ServiceMetadataFrame from inactive services
- Remove ServiceSwitcherFilter and ServiceSwitcherFilterFrame in favor
of plain FunctionFilter instances with closures that check the
strategy's active service directly
- FunctionFilter: add FilterType alias
- FunctionFilter: when direction is None, frames in both directions
are filtered instead of just one
- Add docstrings to ServiceSwitcher and its components
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