Optimize indexed read-only collection writes

[skuirrels]

What changed

• add cached index-based writers for IList<T> and IReadOnlyList<T> collection values
• preserve the exact array and List<T> writer paths
• preserve the enumerable fallback for ambiguous generic element types
• add coverage for ReadOnlyCollection<T>, derived lists, custom read-only lists, vector boundaries, and ambiguous collections

Why

ReadOnlyCollection<T> values previously used the typed enumerable fallback. Repeated interface enumeration boxed an enumerator for every appended collection, causing avoidable managed allocations in high-volume LIST writes.

Benchmark evidence

Controlled BenchmarkDotNet run with 1,000,000 rows and 32 integers per collection:

• ReadOnlyCollection<T> to LIST: 157.3 ms to 150.9 ms, approximately 4.1% faster
• managed allocation: 31,343.4 KB to 91.8 KB, approximately 99.7% lower
• Gen0 collections: 3,000 to zero
• array, exact List<T>, and fixed-array control cases had overlapping confidence intervals and no meaningful winner

Validation

• .NET 8 full suite: 7,030 passed
• .NET 10 full suite: 7,029 passed
• Release solution build succeeded
• git diff --check passed
• final code review found no remaining actionable findings

Release

Intended for stable fork release v1.5.5.4 after hosted checks pass.