[ezkimo]

Fixes #5

Description

Caches the function class discovery in FunctionLoader so repeated load() calls skip filesystem scans and is_subclass_of() checks.

• Static cache (array<string, list<class-string<XPathFunctionInterface>>>) keyed by directory + "\0" + namespace, so separate loader instances with the same configuration share the result. The "\0" separator cannot occur in paths or namespaces, so keys are unambiguous.
• New FunctionLoader::clearCache() to reset the cache in tests or when the functions directory changes at runtime.
• The directory scan moved unchanged into protected discover(); the class is no longer final so tests can count discovery calls — the same trade-off ExpressionNormalizer already makes (issue Cache normalization results in ExpressionNormalizer #3).
• load() / discover() now carry precise @return list<class-string<...>> docblocks, verified by Psalm.
• README documents the caching behaviour and clearCache().

Tests

New tests/Compiler/FunctionLoaderCacheTest.php (8 tests) with autoloadable fixtures under tests/Fixtures/FunctionLoader/Example/:

• repeated load() on the same loader runs discovery only once
• separate loaders with the same directory/namespace share the cache
• different directory or namespace is discovered separately (cache key parts)
• clearCache() forces re-discovery
• black-box check: a file added to the scanned directory stays invisible until clearCache(), then appears
• filter regression: only *Function.php files implementing XPathFunctionInterface are discovered (excludes NotAFunction.php,
  IgnoredHelper.php, notes.md)
• sanity check: the real Compiler/Functions directory still loads CountFunction
• setUp()/tearDown() reset the static cache so tests are order-independent

Compatibility / Risks

Low, as anticipated in the issue:

• load() signature and return type unchanged; public API stays backward compatible.
• Behaviour change: files added/removed from the functions directory mid-process are only picked up after clearCache() — irrelevant for CL runs, now documented in the README.

Verification

• composer test → 65/65 passing
• composer psalm → no errors