Empirically, arduino-cli's --libraries ADDS to the search path — the global
sketchbook is STILL scanned, so the prior commit did NOT isolate reads from the
shared global volume. Point ARDUINO_DIRECTORIES_USER at the scratch sketchbook
instead (its <scratch>/libraries becomes the ONLY user-library dir); cores +
board-manager URLs live in the DATA dir and are untouched.
In-container functest (AVR uno): a cache-only lib compiles via the scope (never
in global); a global-only lib NOT in the manifest is INVISIBLE to the scoped
compile and only recovers via the scan-all retry (manifest_incomplete=True) —
proving the global volume is no longer scanned for a manifest-scoped build.
d3f06265ac