Remove unsafe from DateTimeRawInfo

[EgorBo]

Remove a bit of unsafe code from DateTimeRawInfo + some small perf tweaks to mitigate performance impact from bounds checks.

[EgorBo]

@MihuBot

[EgorBo]

@EgorBot -amd -intel -arm

using System;
using System.Collections.Generic;
using System.Globalization;
using BenchmarkDotNet.Attributes;
using BenchmarkDotNet.Jobs;
using BenchmarkDotNet.Running;

BenchmarkSwitcher.FromAssembly(typeof(DateTimeParsingBenchmarks).Assembly).Run(args);

public class DateTimeParsingBenchmarks
{
    public static IEnumerable<object[]> TestData()
    {
        yield return ["2025-11-10", "yyyy-MM-dd", CultureInfo.InvariantCulture, DateTimeStyles.None];
        yield return ["10/11/2025 14:37:05", "dd/MM/yyyy HH:mm:ss", new CultureInfo("en-GB"), DateTimeStyles.None];
        yield return ["11/10/2025 2:37:05 PM", "M/d/yyyy h:mm:ss tt", new CultureInfo("en-US"), DateTimeStyles.AssumeLocal];
        yield return ["Mon, 10 Nov 2025 14:37:05 GMT", "ddd, dd MMM yyyy HH':'mm':'ss 'GMT'", CultureInfo.InvariantCulture, DateTimeStyles.AdjustToUniversal | DateTimeStyles.AssumeUniversal];
        yield return ["2025-11-10T14:37:05+01:00", "yyyy'-'MM'-'dd'T'HH':'mm':'ssK", CultureInfo.InvariantCulture, DateTimeStyles.RoundtripKind];
    }

    [Benchmark]
    [ArgumentsSource(nameof(TestData))]
    public DateTime TryParseExact_WithCases(string text, string format, CultureInfo culture, DateTimeStyles styles)
        => DateTime.TryParseExact(text, format, culture, styles, out var dt) ? dt : default;
}

[Copilot]

Pull Request Overview

This PR optimizes the DateTimeParse class by converting data structures to more efficient representations and removing unsafe code. The changes improve memory layout and performance while maintaining the same functionality.

Key changes:

• Converts the state machine lookup table from a jagged 2D array (DS[][]) to a flat 1D ReadOnlySpan<DS> with a helper method for indexed access
• Adds explicit base types to internal enums (TM, DS, DTSubStringType) for better memory efficiency
• Removes unsafe code by replacing int* with InlineArray3<int> in DateTimeRawInfo

[EgorBo]

PTAL @stephentoub @tannergooding

I removed an unsafe block and had to perform a few micro-optimizations to mitigate perf impact from bounds checking.

1. converted DS[][] to an RVA single-dimension field
2. marked a few structs as Auto-layout so they can shrink in size and various initialization/copies are sligtly cheaper.

[stephentoub]

LGTM if there are no regressions

[EgorBo]

/ba-g certificate issues