Resolution caches are process-wide but depend on the per-thread context #33

All line numbers are at main 1fcdf12.

Problem

Two caches hold answers that depend on the active context but are owned by the module, not by the context:

• _mro_resolution_cache in dual_axis_resolver.py:20
• the extract_all_configs cache in context_manager.py

Consequences:

1. config_context clears both on every entry and every exit (context_manager.py:241-243, :251-252). The context is per thread or task; the caches are process-wide. One thread entering a scope empties the cache another thread is using. Nested contexts clear it at every level, so inside a deep stack the cache is rebuilt repeatedly.
2. The cache key uses id(config_instance) (dual_axis_resolver.py:162). An id is only unique while the object is alive. Correctness depends on every path that can drop a config instance also clearing the cache; invalidate_mro_cache_for_field removes entries by type and field name only.

Wanted

The cached answers belong to the context value they were computed from: stored on, or keyed by, the merged config object itself, so they disappear with it and no clear call is needed on entry or exit. Keys hold the object (or a weak reference), not its id.

Acceptance

config_context contains no cache-clearing calls. A test resolves a field in one thread, enters and exits a context in a second thread, and shows the first thread's cached entry is still used. No cache key contains id(...).