Cache custom attribute lookups per property descriptor

[lahma]

GetCustomAttribute<T>() re-ran reflection (Attribute.GetCustomAttributes) on every call, once per property, on both the serialize and deserialize paths — profiled (ultra) at ~11% inclusive time. This memoizes the result per descriptor in a lazily-created ConcurrentDictionary, preserving the exact lookup and caching absent-attribute (null) results so the common no-attribute case is not re-reflected. Attributes are immutable metadata, so behavior is unchanged — all 1979 unit + spec tests pass; builds on net10.0/net8.0/netstandard2.0/netstandard2.1/net47.

Same-session A/B (net10.0, DefaultJob, MemoryDiagnoser):

Benchmark	Baseline	This PR	Δ time
Alias.DeserializeWithAliases	2,358.8 µs	1,458.5 µs	−38.2%
Alias.SerializeWithAliases	972.1 µs	651.5 µs	−33.0%
ObjectGraph.SerializeTyped	37.55 ms	27.48 ms	−26.8%
SerializationBenchmarks.Serializer	21.79 ms	17.26 ms	−20.8%
ObjectGraph.DeserializeTyped	51.13 ms	41.25 ms	−19.3%
ObjectGraph.RoundtripTyped	77.91 ms	65.56 ms	−15.9%
ObjectGraph.DeserializeUntyped	41.99 ms	42.05 ms	~0%
YamlStream.LoadLarge (parse control)	60.50 ms	59.17 ms	~0%

Allocations unchanged (the per-descriptor dictionary is allocated once, not per operation). Benchmarks live in #1119 — merge that first to reproduce.