Add Fusion hot path micro benchmarks

[michaelstaib]

No description provided.

[Copilot]

Pull request overview

This pull request adds a suite of BenchmarkDotNet micro-benchmarks targeting Fusion execution hot paths, with baseline-vs-candidate variants and equivalence checks to validate candidate behavior.

Changes:

• Adds benchmarks for MetaDb row decode elimination / row-carrying enumeration and fused decode paths.
• Adds benchmarks for reducing repeated selection/type-shape work and child selection-set lookup work.
• Adds benchmarks for lowering JSON/name scanning and variable serialization overhead on merge paths.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/SourceEnumeratorRowCarryBenchmark.cs	Benchmarks duplicate source row decodes during object/array enumeration and a row-carrying alternative.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/SelectionTypeShapeBenchmark.cs	Benchmarks per-completion type-shape derivation vs precomputed per-selection shape reads.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/SelectionChildMapBenchmark.cs	Benchmarks per-property child selection set lookup vs leaf-gated indexed memoization.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/PropertyNameLengthPrefilterBenchmark.cs	Benchmarks property-name scan with a quoted-length prefilter to avoid unnecessary name reads.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/ForwardedVariableBlitBenchmark.cs	Benchmarks per-entry forwarded-variable reserialization vs prefix capture + raw splice (“blit”).
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/CompositeContextFusedDecodeBenchmark.cs	Benchmarks redundant composite target row decodes vs fused single-decode context acquisition.
src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/AbstractSelectionSetCacheBenchmark.cs	Benchmarks abstract selection-set resolution via operation dictionary vs small copy-on-write cache.

Comments suppressed due to low confidence (1)

src/HotChocolate/Fusion/benchmarks/Fusion.Execution.Benchmarks/SourceEnumeratorRowCarryBenchmark.cs:246

• This throws InvalidOperationException without a message. If this guard ever trips (e.g., if the benchmark logic drifts), diagnosing the failure will be harder than necessary. Use a message consistent with similar guards elsewhere in these benchmarks (and product code).

        if (snapshot._parent is null || snapshot._row.TokenType != JsonTokenType.StartArray)
        {
            throw new InvalidOperationException();
        }

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