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.

[fallenmi]

The new recency queue is mutated before self._cache[item] establishes that the key exists. MutableMapping.__contains__ calls __getitem__, so an ordinary miss corrupts the eviction order:

d = DirCache(max_paths=2)
d["a"] = 1
d["b"] = 2
assert "missing" not in d
d["c"] = 3
assert dict(d._cache) == {"b": 2, "c": 3}

On exact head 4544763, the final cache is only {"c": 3} and _q is OrderedDict({"missing": None, "c": None}): the ghost key makes the next insertion evict both valid entries. The exact base fails the normal LRU oracle while this head fixes it, but this absent-key oracle still fails. Please read the cached value, allowing KeyError to happen, before refreshing _q, and cover missing and expired lookups. I also ran the changed target plus test_core.py: 62 passed, 6 skipped; the diff check and compile check are clean.

Reviewed with Codex (GPT-5); the exact base/head oracles and listed tests were run locally.