Reduce ambiguity about the metadata's state

[kahrendt]

Replace the metadata role's empty-string approach to clearing old metadata with the new ServerMetadataStateDelta struct to reduce ambiguity for metadata's tri-state absent, null, or value.

This is a behavioral breaking change. The server clears old metadata states with a nullopt instead of an empty string. Updating the state where progress: null now properly clears the current progress info that before was never actually possible to clear.

[Copilot]

Pull request overview

This PR changes how server-sent metadata updates represent “clear” operations by introducing a dedicated wire-level delta type (ServerMetadataStateDelta) with tri-state fields (absent vs explicit null vs value), removing the previous empty-string clearing behavior to reduce ambiguity in merged metadata state.

Changes:

• Introduced ServerMetadataStateDelta (tri-state per field) and updated server/state parsing to populate it.
• Updated metadata delta application logic so explicit JSON null clears merged fields (including progress).
• Updated docs and the TUI example to encourage overwriting mirrored display state each on_metadata() so clears propagate.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/protocol.cpp	Adds tri-state parsing helpers, parses metadata into ServerMetadataStateDelta, and applies deltas to merged ServerMetadataStateObject with explicit-null clears.
src/protocol_messages.h	Introduces ServerMetadataStateDelta, updates ServerStateMessage to carry metadata deltas, and updates delta-merge API signature/docs.
src/metadata_role.cpp	Updates metadata role to accept/merge wire deltas in the shadow slot and apply them to the merged metadata state when due.
src/metadata_role_impl.h	Updates shadow slot type and method signature to use ServerMetadataStateDelta (and adds needed include).
examples/tui_client/main.cpp	Updates metadata listener to always overwrite title/artist/album (so server clears propagate).
docs/internals.md	Updates internal shadow-slot documentation to reflect metadata delta type and clear behavior.
docs/integration-guide.md	Adds guidance about nullopt representing both “never provided” and “explicitly cleared”, advising overwrite-on-update for UI mirroring.

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