Variable REPLAYGAIN_SOURCE_MODEConst

Numeric identifiers for ReplaygainSourceModeName, mirroring the host's source-mode enum. Compare the mode that config.getReplaygainMode reports against entries in this dictionary instead of remembering the literals:

const r = await config.getReplaygainMode();
if (r.mode === REPLAYGAIN_SOURCE_MODE.track) { ... }

auto is intentionally omitted: it is an input-only alias of byPlaybackOrder.