The persistent per-target build/ caches ESP-IDF's cmake configuration, ninja's
incremental graph and ccache-backed objects, all assuming a stable component
set. When consecutive compiles on the same dir have a DIFFERENT resolved
user_libs set (a different project/user, or a different library manifest) that
cache is inconsistent and produces two real failures:
- cmake reconfigure intermittently fails ('cmake configure failed') even
though each manifest compiles fine on a clean dir;
- ninja/ccache reuse a previous compile's objects/headers, letting a
now-absent library slip through as a false-positive success against a lib
the current sketch/manifest no longer includes.
Fingerprint the materialized user_libs/ (sorted relative paths + sizes) and,
when it changes vs the last compile on this dir, wipe build/ to force a clean
configure. ccache (enabled) refills the objects so the rebuild stays cheap.
No-op on the ephemeral path and the first compile. Fixes both symptoms; will be
superseded by the fully ephemeral per-compile workspace (P1).
161deb93e2