add 7bit key

[jamesmoore]

Add 64x7bit key using 4 uints instead of 8

[Copilot]

Pull request overview

Introduces a new solver variant (AutogramBytesNoStringsV5f) that uses a more compact 7-bit-packed history key to reduce memory usage, and wires it into the test suite and benchmarks for comparison with prior versions.

Changes:

• Add ByteHistoryKey7Bit128 (7-bit packing into 4×uint) and use it in new solver AutogramBytesNoStringsV5f.
• Add a new unit test covering AutogramBytesNoStringsV5f.
• Extend benchmarks to include V5f and change which benchmark is marked as the baseline.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
AutogramTest/AutogramTest.cs	Adds a new [Fact] to run the V5f solver against the standard autogram test.
AutogramBenchmark/AutogramBenchmark.cs	Adds setup/cleanup + benchmark entry for V5f; baseline benchmark designation adjusted.
Autogram/ByteHistoryKey7Bit128.cs	New packed key struct intended to store history keys more compactly.
Autogram/AutogramBytesNoStringsV5f.cs	New solver variant that swaps the history key type to the new packed key.

Comments suppressed due to low confidence (1)

Autogram/ByteHistoryKey7Bit128.cs:24

• stackalloc uint[4] does not guarantee zero-initialized memory; chunks may contain garbage and the subsequent |= operations can produce nondeterministic keys, breaking HashSet lookups (missed hits / unbounded growth). Clear the span (e.g., chunks.Clear()) or explicitly initialize the 4 uints before OR-ing in values.

            Span<uint> chunks = stackalloc uint[4];
            for (var i = 0; i < values.Length; i++)
            {
                var value = values[i];
                ArgumentOutOfRangeException.ThrowIfGreaterThanOrEqual(value, (byte)128);

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