Supersedes the mid-_compile_in_dir build/ wipe (161deb9): wiping build/ AFTER
materializing libs but right before cmake left ESP-IDF's config half-regenerated
during the fallback's scoped->scan-all double-compile, intermittently failing
with 'sdkconfig.h: No such file'.
Instead fold the effective library set (the manifest, else the sketch's
non-core external includes) into the per-attempt build-dir hash, so a changed
lib set — or the scan-all fallback after a scoped attempt — resets the
persistent build/ at _prepare_persistent_project_dir time (before any cmake).
That is the existing, well-tested early-wipe path, so the configure is always
clean. Same lib set across compiles keeps the warm ccache/ninja cache; core-only
sketches share one dir (core headers filtered out of the token) so they never
trigger a spurious wipe.
Fixes both the original cross-compile staleness (intermittent cmake-configure
failures + stale-object false positives) and the fallback regression.
33fbc03429