Add PrecompileTools workload for faster precompilation

[AhmedSalih3d]

Motivation

• Reduce time-to-first-simulation by precompiling representative, hot-code paths so subsequent using SPHExample is faster.
• Warm core kernel and equation code across platforms to make package compilation more efficient and predictable.

Description

• Add PrecompileTools to Project.toml and its compat entry to track the new dependency.
• Add a @setup_workload / @compile_workload block in src/SPHExample.jl that exercises core functions including Wᵢⱼ, ∇Wᵢⱼ, tensile_correction, EquationOfStateGamma7, ConstructGravitySVector, Pressure!, and LimitDensityAtBoundary!.
• Import StaticArrays in the module to build small example inputs used by the precompile workload and add a short note to README.md describing precompile behavior.

Testing

• Executed julia -e 'using Pkg; Pkg.activate("."); Pkg.add("PrecompileTools")' to add the dependency and update Project.toml.
• No automated test suite was run for this change and no precompile cache generation was executed as part of the PR.

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Codex Task