Make the benchmark suite self-contained (drop the StableRNGs dependency)

The `Benchmarks` CI job errored with `ArgumentError: Package StableRNGs not
found in current path`. AirspeedVelocity's `benchpkg` benchmarks each revision
in a bare environment containing only the package under test (it does not read
`benchmark/Project.toml` — `project_toml` is `nothing`), so `using StableRNGs`
in `benchmark/benchmarks.jl` could never resolve.

Replace StableRNGs with a tiny vendored SplitMix64 stream (`_bench_rand`)
mapped to Float64 in [0, 1). It is deterministic across Julia versions and
branches — all the base-vs-PR comparison needs — and pulls in no packages, so
the suite runs in the environment benchpkg builds.

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

Verified in a bare env (only FindFirstFunctions + BenchmarkTools, no
StableRNGs): the suite loads, builds all groups, 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/benchmarks.jl

@@ -5,36 +5,52 @@
 
 using BenchmarkTools
 using FindFirstFunctions
-using StableRNGs
 
 const SUITE = BenchmarkGroup()
 
+# Dependency-free deterministic RNG (SplitMix64 mapped to Float64 in [0, 1)).
+# AirspeedVelocity benchmarks each revision in a bare environment containing
+# only the package under test, so the suite must not pull in extra packages;
+# this stream is deterministic across Julia versions and branches, which is
+# all the base-vs-PR benchmark comparison requires.
+function _bench_rand(n::Integer, seed::Integer)
+    out = Vector{Float64}(undef, n)
+    s = UInt64(seed)
+    @inbounds for i in 1:n
+        s += 0x9e3779b97f4a7c15
+        z = s
+        z = (z ⊻ (z >> 30)) * 0xbf58476d1ce4e5b9
+        z = (z ⊻ (z >> 27)) * 0x94d049bb133111eb
+        z = z ⊻ (z >> 31)
+        out[i] = (z >> 11) * (1.0 / (UInt64(1) << 53))
+    end
+    return out
+end
+
 # Helper: build a sorted Float64 vector with the given pattern.
 function _make_v(kind, n, seed = 2026)
-    rng = StableRNG(seed)
     return if kind === :uniform
         collect(range(0.0, Float64(n); length = n))
     elseif kind === :logspaced
         collect(exp.(range(0.0, log(1.0e6); length = n)))
     elseif kind === :random_sorted
-        sort!(rand(rng, n) .* n)
+        sort!(_bench_rand(n, seed) .* n)
     else
         error("unknown v kind $kind")
     end
 end
 
 function _make_q(v, m, kind, seed = 2027)
-    rng = StableRNG(seed)
     return if kind === :dense
         collect(range(first(v), last(v); length = m))
     elseif kind === :sparse
-        sort!(first(v) .+ rand(rng, m) .* (last(v) - first(v)))
+        sort!(first(v) .+ _bench_rand(m, seed) .* (last(v) - first(v)))
     elseif kind === :clustered
         j = max(1, length(v) ÷ 4)
         lo = v[j]
         hi = v[min(j + 1, length(v))]
         span = hi - lo
-        sort!(lo .+ rand(rng, m) .* max(span, 1.0))
+        sort!(lo .+ _bench_rand(m, seed) .* max(span, 1.0))
     else
         error("unknown q kind $kind")
     end