chore: scoped public span params + CountLeadingZeros test/benchmark cleanup

[unsafePtr]

Summary

Two small, dependency-free cleanups to the Base58 library:

1. Defensive scoped on public span parameters — annotate the public Encode/Decode ReadOnlySpan/Span parameters with scoped (mirrors MessagePack-CSharp #2271). Documents and compiler-enforces that the API never captures caller buffers. No codegen change, non-breaking.

2. CountLeadingZeros test + benchmark cleanup — retarget the leading-zero tests from now-deleted internals to the public CountLeadingZeros contract (plus an all-zeros case and deterministic inputs), and add an IndexOfAnyExcept variant to CountLeadingZerosBenchmark that records the measured result: the BCL span-scan is ~27% slower than the hand-rolled SIMD path, so it was measured and not adopted.

Parked for later

A separate SIMD / table-layout exploration (flattening the uint[][] lookup tables, TensorPrimitives.Dot) is kept on a local experiment/base58-simd-tables branch. TensorPrimitives.Dot<ulong> showed a real decode kernel win but requires an ML-math (System.Numerics.Tensors) dependency, and firedancer itself keeps the matrix-multiply scalar — so it needs more end-to-end evaluation before adopting. Not part of this PR.

Testing

• 123 unit tests pass (1 skipped: local-only big-endian test).
• Benchmark project builds clean.