Add benchmark/Project.toml so the AirspeedVelocity suite resolves its deps

The benchmark workflow errored with `ArgumentError: Package StableRNGs not
found in current path`: `benchmark/benchmarks.jl` does `using StableRNGs`,
but there was no `benchmark/Project.toml`, so the benchmark environment
benchpkg builds had no StableRNGs. Add the environment with the suite's
dependencies (BenchmarkTools, StableRNGs, plus FindFirstFunctions for the
package-under-test slot).

Also fix the workflow path filter: it watched `bench/**`, but the suite
lives in `benchmark/**`, so benchmark-only changes never triggered the job.

Verified locally: `julia --project=benchmark benchmark/benchmarks.jl` loads
all dependencies, builds the SUITE (equality / per_query / batched /
props_construct), and runs a benchmarkable.

Co-Authored-By: Chris Rackauckas <accounts@chrisrackauckas.com>
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01FvBXVKGujjeaCB3iLwsDeG

Author: ChrisRackauckas

.github/workflows/Benchmark.yml

@@ -4,7 +4,7 @@ on:
   pull_request:
     paths:
       - "src/**"
-      - "bench/**"
+      - "benchmark/**"
       - "Project.toml"
       - ".github/workflows/Benchmark.yml"
   workflow_dispatch:

benchmark/Project.toml

@@ -0,0 +1,8 @@
+[deps]
+BenchmarkTools = "6e4b80f9-dd63-53aa-95a3-0cdb28fa8baf"
+FindFirstFunctions = "64ca27bc-2ba2-4a57-88aa-44e436879224"
+StableRNGs = "860ef19b-820b-49d6-a774-d7a799459cd3"
+
+[compat]
+BenchmarkTools = "1"
+StableRNGs = "1"