Settings that can be overridden per source. Stored in
AppConfig::source_overrides keyed by source_id. When a section
starts, the effective settings are computed by merging the saved
override (if any) over the global defaults.
Variant of source_id keyed by an explicit device name. Preferred
over source_id when the actual selected device is known (the
generic Microphone/System variants collapse multiple devices to
default, which would conflate per-device overrides).
Stable identifier for a source used to key source_overrides. Format:
device:input:<name> / device:output:<name> /
app:<bundle-or-name>@device:<device>.