Add allocation tests for core utility functions

[ChrisRackauckas-Claude]

Summary

• Add allocation tests for key internal utility functions in BoundaryValueDiffEqCore: interval, recursive_flatten!, recursive_unflatten!, __maybe_matmul!, and diff!
• Tests verify these functions do not allocate after warmup
• The original code changes (removing array comprehensions in loss functions) were already merged to master through other PRs, so this PR now only contains the new test file

Changes

• test/misc/allocation_tests.jl (new file) — @testitem with 5 @testset blocks testing zero allocations for in-place utility functions

Test plan

• Pkg.test() passes locally (all 8 misc test items pass, 75 tests pass)
• Runic formatting check passes

[github-actions]

Benchmark Results

Click to check benchmark results

	master	3cba24f...	master / 3cba24f...
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK2()	0.318 ± 0.02 s	0.306 ± 0.02 s	1.04 ± 0.095
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK3()	9.99 ± 2 ms	9.92 ± 0.43 ms	1.01 ± 0.21
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK4()	2.5 ± 0.096 ms	2.47 ± 0.085 ms	1.01 ± 0.052
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK5()	7.31 ± 1.9 ms	7.31 ± 1.8 ms	0.999 ± 0.36
Simple Pendulum/IIP/BoundaryValueDiffEqMIRK.MIRK6()	1.33 ± 0.35 ms	1.31 ± 0.077 ms	1.01 ± 0.28
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = false)	1.77 ± 0.99 ms	1.75 ± 1 ms	1.01 ± 0.81
Simple Pendulum/IIP/MultipleShooting(10, Tsit5; grid_coarsening = true)	2.97 ± 0.95 ms	2.95 ± 0.97 ms	1.01 ± 0.46
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.0759 ± 0.016 s	0.0706 ± 0.016 s	1.08 ± 0.34
Simple Pendulum/IIP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.0931 ± 0.025 s	0.092 ± 0.023 s	1.01 ± 0.37
Simple Pendulum/IIP/Shooting(Tsit5())	0.239 ± 0.075 ms	0.239 ± 0.073 ms	1 ± 0.44
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK2()	0.501 ± 0.011 s	0.464 ± 0.0086 s	1.08 ± 0.031
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK3()	12.8 ± 7.8 ms	12.2 ± 7.1 ms	1.05 ± 0.89
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK4()	2.87 ± 0.28 ms	2.83 ± 0.24 ms	1.01 ± 0.13
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK5()	8.56 ± 1.2 ms	8.3 ± 1.2 ms	1.03 ± 0.21
Simple Pendulum/OOP/BoundaryValueDiffEqMIRK.MIRK6()	1.53 ± 0.24 ms	1.48 ± 0.23 ms	1.03 ± 0.23
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = false)	3.18 ± 3.1 ms	3.05 ± 3.2 ms	1.04 ± 1.5
Simple Pendulum/OOP/MultipleShooting(10, Tsit5; grid_coarsening = true)	5.49 ± 6.8 ms	5.17 ± 5.8 ms	1.06 ± 1.8
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = false)	0.124 ± 0.016 s	0.124 ± 0.0098 s	1 ± 0.15
Simple Pendulum/OOP/MultipleShooting(100, Tsit5; grid_coarsening = true)	0.16 ± 0.076 s	0.149 ± 0.068 s	1.08 ± 0.71
Simple Pendulum/OOP/Shooting(Tsit5())	0.535 ± 0.23 ms	0.531 ± 0.3 ms	1.01 ± 0.71
time_to_load	4.62 ± 0.027 s	4.5 ± 0.024 s	1.03 ± 0.0082

### Benchmark Plots A plot of the benchmark results has been uploaded as an artifact to the workflow run for this PR. Go to "Actions"->"Benchmark a pull request"->[the most recent run]->"Artifacts" (at the bottom).