A scoped ESP32 compile can now resolve libraries from a per-compile directory
provided by an overlay (the manifest's libs symlinked from a content-addressed
cache, with a legacy-dir fallback) instead of the single shared global volume.
- core/hooks.py: register_materialize_library_scope / materialize_library_scope
(no-op default -> None, so the OSS image keeps its single scan-all dir).
- espidf_compiler: _attempt(allowed) calls the hook, folds the returned content
token into the build-variant eff_hash (a content change gets a clean build
dir), passes libraries_dir to _compile_in_dir (arduino_libs = libraries_dir or
_find_arduino_libraries_dir()), and removes the throwaway dir after. The
graceful scan-all fallback (allowed=None) keeps using the default dir, so the
worst case of any materializer failure is fall-back-to-legacy (no break).
810d8e4b51