Speed up ULID parsing, generation, and encoding; fix related correctness bugs

[Seramis]

Description

Performance-focused pass over the core Ulid type's hot paths (parsing, string encoding/decoding, generation, timestamp arithmetic, locking), bundled with several correctness fixes surfaced along the way. README.md updated with latest benchmark results.

Performance:

• Base32 encoding/decoding now uses two 64-bit loads instead of 16 byte loads, with the inverse table stored as RVA data.
• MinAt/MaxAt/MaxValue are built without static byte arrays.
• Generator state is kept in numeric order, removing byte swaps from the same-millisecond increment path.
• Timestamp arithmetic uses unsigned division for "now", and a single 64-bit load (instead of 6 byte loads) to read a ULID's timestamp.
• Lock contention: switched to TTAS spinning (spinners no longer steal the lock's cache line via CAS) and padded generator state against neighboring objects.
• Scalar GUID shuffle no longer allocates.
• Dropped AggressiveOptimization letting tiered PGO optimize hot paths and removing per-TFM #if blocks. Moved SkipLocalsInit to module-level.
• GenerationOptions copies now get independent lock/state instead of sharing it with the source. Monotonicity is validated via range check (More AOT friendly), and the random increment mask is precomputed when set.

Correctness fixes:

• Parse accepted invalid, non-ASCII, and overflowing input, and TryParse relied on exceptions internally. Both are fixed, with characters decoded and validated one by one.
• IsValid(ReadOnlySpan<byte>) only checked length (16 bytes) and didn't validate UTF-8 content like Parse/TryParse do. Validation rules are now documented.
• New(timestamp, random) didn't validate the random span length, allowing reads past the end for shorter spans.
• Fixed GC holes in AsByteSpan and a lock scope where spans from untracked pointers could outlive a GC move. netstandard2.0's AsByteSpan now copies.
• UlidJsonConverter didn't support escaped JSON strings.
• netstandard2.0 RNG shims didn't return rented arrays.
• Fixed a benchmark bug where GetNextIndex only alternated between the first two entries and all buffers were shared, so most compared pairs were identical.

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Other (please specify): Performance optimizations, with accompanying correctness fixes

Checklist

• The PR is submitted to the correct branch (main).
• My code follows the project's coding style. (.editorconfig)
• I have commented my code, particularly in hard-to-understand areas and public interfaces.
• I have added or updated tests for the changes I made.
• All new and existing tests passed.
• I have updated the documentation where applicable.

Additional Notes

No public API breaking changes.
This is primarily an internal hot-path rewrite plus validation fixes that tighten previously-loose input handling (Parse, TryParse, IsValid, New(timestamp, random)).