The last paths that read the shared global /root/Arduino/libraries: a compile
with NO manifest (libraries=null — any from-scratch/anon sketch; the manifest is
never auto-derived from #includes) and the incomplete-manifest scan-all retry
(which re-enters compile unscoped). Both fell through to the global dir,
bypassing the cache entirely (a cached lib still failed when global was gone).
Now, when no scope is materialized, point the library search at the cache: the
cache root is itself a valid Arduino libraries dir (each <name@ver-sha> child is
a library), exposed via env (pro overlay sets them):
- arduino-cli: ARDUINO_DIRECTORIES_USER = VELXIO_FALLBACK_SKETCHBOOK (whose
libraries/ -> cache root) when scope_dir is None.
- ESP-IDF: _find_arduino_libraries_dir() prefers VELXIO_FALLBACK_LIBRARIES_DIR.
Unset (OSS self-host) -> legacy global, unchanged.
Also strip a trailing @version from manifest names (_bare_lib_names): norm_name
fused 'ArduinoJson@6.21.5' -> 'arduinojson6215' (cache miss -> global scan-all);
the per-board boards_json manifest is the path that still carries @version.
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