Library manifests are now PER-BOARD (each board carries its own velxio.json),
so two boards in one project can use different (even conflicting) libraries
without clashing — the multi-board extension of the no-clash guarantee.
- board.libraries on BoardInstance + serialisableBoard: rides in boards_json,
so it round-trips, dirty-checks, autosaves and restores natively. This also
removes the load-restore hacks (useLibraryManifestStore + applyProjectManifest
deleted): the manifest is plain board state.
- loadProjectState now restores per-board boardOptions/spiffsFiles/libraries
(it previously dropped them).
- EditorToolbar single + compile-all send the COMPILING board's libraries.
- Backend compile.py prefers the client's per-board request.libraries; the
project-level libraries_json (now the union of all boards) is the fallback.
- buildLoadPayload migrates pre-per-board projects: seed each board with the
project union so they keep compiling scoped.
- Library Manager 'In project' tab edits the ACTIVE board's velxio.json (shows
the board name) and the add field is now an autocomplete (installed libs +
index search) so users pick from a list instead of typing names.
Deletes useLibraryManifestStore.ts + applyProjectManifest.ts.
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