Add AirspeedVelocity benchmarking CI

.github/workflows/benchmark.yml

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+name: Benchmark this PR
+
+on:
+  pull_request:
+    branches: [main]
+    paths-ignore: ['docs/**']
+
+permissions:
+  pull-requests: write
+  contents: read
+
+jobs:
+  benchmark:
+    runs-on: ubuntu-latest
+    timeout-minutes: 30
+    strategy:
+      fail-fast: false
+      matrix:
+        version: ["1", "lts"]
+    env:
+      # Force consistent Julia depot path for self-hosted runners
+      JULIA_DEPOT_PATH: ~/.julia
+    steps:
+      - uses: MilesCranmer/AirspeedVelocity.jl@action-v1
+        with:
+          julia-version: ${{ matrix.version }}
+          script: "benchmark/benchmarks.jl"
+          extra-pkgs: "SparseArrays,StableRNGs"
+          job-summary: true

benchmark/benchmarks.jl

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+using SparseMatrixIdentification, BenchmarkTools
+using StableRNGs, SparseArrays
+
+const SUITE = BenchmarkGroup()
+const rng = StableRNG(123)
+
+n = 2000
+A_sparse = sprand(rng, n, n, 0.005)
+# Banded sparse matrix
+A_band = spdiagm(-1 => rand(rng, n - 1), 0 => rand(rng, n), 2 => rand(rng, n - 2))
+# Block-banded
+blk = 50
+A_blk = spdiagm(
+    -blk => rand(rng, n - blk), 0 => rand(rng, n), blk => rand(rng, n - blk)
+)
+
+# =============================================================================
+# Structure detection
+# =============================================================================
+
+SUITE["detect"] = BenchmarkGroup()
+
+SUITE["detect"]["getstructure_sparse"] = @benchmarkable getstructure($A_sparse)
+SUITE["detect"]["getstructure_banded"] = @benchmarkable getstructure($A_band)
+SUITE["detect"]["getstructure_blockbanded"] = @benchmarkable getstructure($A_blk)
+SUITE["detect"]["sparsestructure"] = @benchmarkable sparsestructure(
+    $A_sparse, 0.5
+)
+SUITE["detect"]["sparsestructure_dense"] = @benchmarkable sparsestructure(
+    $(sparse(rand(rng, 500, 500))), 0.5
+)