Cache ObjectAccess::forContext() using ScopedCache

[veewee]

Summary

ObjectAccess::forContext() rebuilds the full property mapping for a complex type on every encode/decode call: iterates all properties from metadata, detects encoders for each, creates encoder/decoder lenses, and builds isos. This is the most expensive operation on the hot path.

Now cached per (registry, namespace, typeName, bindingUse) using ScopedCache (introduced in #56).

Cache key

• namespace|typeName: determines which type and properties are resolved from metadata
• bindingUse: affects the iso closures (xsi:type attributes in encoded mode, XMLWriter bypass in literal)
• registry: is the WeakMap scope (separate cache per driver instance)

Why bindingUse is in the key

The same complex type used with literal vs encoded binding produces different isos: encoded mode adds xsi:type attributes via XsiAttributeBuilder, literal mode may bypass XMLWriter entirely. A test verifies this by comparing the XML output of the same type with both bindings.

Impact (local benchmark)

Subject	Main	Feature	Delta
Encode Literal x500	59ms	44ms	-26%
Encode Encoded x500	82ms	73ms	-11%
Decode Literal x500	81ms	65ms	-20%
Decode Encoded x500	116ms	102ms	-12%

Test plan

• Full test suite passes (560 tests, 3 new)
• Psalm clean
• CS fixer clean
• Benchmarked against main
• Test: different bindingUse produces different ObjectAccess (different isos)
• Test: different registries don't share cache
• Test: same context returns cached instance