Add benchmark/Project.toml so the benchmark suite resolves StableRNGs

[ChrisRackauckas-Claude]

Please ignore until reviewed by @ChrisRackauckas. Draft — opened by an agent.

Problem

The Benchmarks CI job fails on every PR that touches src/** (seen on #100, #101):

ERROR: LoadError: ArgumentError: Package StableRNGs not found in current path.

benchmark/benchmarks.jl does using StableRNGs, but there was no benchmark/Project.toml. AirspeedVelocity builds a temp environment for each benchmarked revision and, when that revision's checkout contains benchmark/Project.toml, copies it in as the environment (it auto-adds BenchmarkTools/JSON3 and the package under test, but nothing else). With no such file, StableRNGs never resolves.

Fix

Add benchmark/Project.toml declaring StableRNGs. That's all AirspeedVelocity needs (it supplies BenchmarkTools/JSON3 and the package itself).

Verification

Replicated AirspeedVelocity's env construction locally — copy benchmark/Project.toml into a fresh env, Pkg.develop the package, Pkg.add(["BenchmarkTools","JSON3"]) — then ran the unmodified benchmark/benchmarks.jl: StableRNGs resolves, the suite builds all groups (equality / per_query / batched / props_construct), and a benchmarkable runs.

Note on CI

This PR's own Benchmarks check can't go green (and won't trigger): the workflow runs benchpkg --rev=main,<PR> --bench-on=main, so it benchmarks using main's checkout, which won't have benchmark/Project.toml until this merges. The fix takes effect on the base branch — subsequent PRs' benchmark runs then resolve cleanly.

(Separately: the Benchmark.yml path filter watches bench/**, but the suite lives in benchmark/**; a one-line fix, left out here to keep this PR to just the Project.toml.)

[ChrisRackauckas-Claude]

Note on the failing Benchmarks check — expected, and it clears on merge.

The SciML benchmark workflow invokes:

benchpkg "FindFirstFunctions" --rev=main,<PR sha> --bench-on=main

--bench-on=main makes AirspeedVelocity use main's benchmark/benchmarks.jl to benchmark both revisions. Since main's script still has using StableRNGs, the run fails at load (Running benchmarks for FindFirstFunctions@main: … Package StableRNGs not found) — before this PR's fixed script is ever used. So the benchmark check on this PR cannot pass until the fix is on the base branch; it's a chicken-and-egg for a PR that fixes the benchmark job itself.

The fix is verified out-of-band: in a bare environment (only FindFirstFunctions + BenchmarkTools, no StableRNGs) benchmark/benchmarks.jl on this branch loads, builds all groups, and runs a benchmarkable. Once merged, subsequent PRs' benchmark runs (which will --bench-on=main against the fixed script) resolve cleanly.

[ChrisRackauckas-Claude]

Updated: switched from vendoring an RNG to simply adding benchmark/Project.toml with StableRNGs — confirmed AirspeedVelocity auto-detects and uses a benchmarked revision's benchmark/Project.toml (Utils.jl: "Found Project.toml at …" → copies it into the env). My earlier "benchpkg ignores it" note was mistaken; those runs used --bench-on=main where main had no such file. This supersedes the previous comment.