fix: use real LRU eviction in DirCache

[aryansk]

Fixes #2095

Problem

DirCache uses a functools.lru_cache as an eviction helper, but lru_cache calls its wrapped function only on cache misses — never when an old key is evicted from its recency index. As a result:

1. Adding entries beyond max_paths never removes old entries from _cache (unbounded growth).
2. Reading an evicted key pops it from _cache lazily and raises KeyError.
3. __iter__ filters through __getitem__, so iterating destroys the entire cache and returns [].

Change

Track recency with an OrderedDict instead:

• on insert, evict the least-recently-used entry (from both _cache and _times) when over max_paths;
• on access, refresh the recency order without popping the listing;
• __delitem__ and clear() keep the recency index consistent.

Testing

Added fsspec/tests/test_dircache.py covering eviction, iteration stability, recency refresh, delete, and expiry. With the fix: 160 passed in the cache test modules; the remaining failures/errors are pre-existing missing-optional-dependency (aiohttp) import issues.

Notes

• max_paths is currently undocumented in the class docstring (only in the __init__ docstring); I left that unchanged to keep the diff focused.