feat(bench): add safety number & epoch rotation benchmarks, CI workflow

Add safety_number benchmark to crypto_benchmarks.rs, epoch rotation
(propose_self_update + commit) benchmark to mls_operations.rs, expand
add_member group sizes to include 100, and add .github/workflows/bench.yml
that runs Criterion benchmarks and uploads HTML reports as artifacts.

crates/quicproquo-core/benches/crypto_benchmarks.rs

@@ -8,7 +8,7 @@
 
 use criterion::{black_box, criterion_group, criterion_main, BenchmarkId, Criterion};
 
-use quicproquo_core::{IdentityKeypair, padding};
+use quicproquo_core::{compute_safety_number, IdentityKeypair, padding};
 
 // ── Identity keypair benchmarks ──────────────────────────────────────────────
 
@@ -127,6 +127,17 @@ fn bench_padding(c: &mut Criterion) {
     group.finish();
 }
 
+// ── Safety number benchmarks ─────────────────────────────────────────────────
+
+fn bench_safety_number(c: &mut Criterion) {
+    let key_a = [0x1au8; 32];
+    let key_b = [0x2bu8; 32];
+
+    c.bench_function("safety_number", |b| {
+        b.iter(|| black_box(compute_safety_number(black_box(&key_a), black_box(&key_b))));
+    });
+}
+
 criterion_group!(
     benches,
     bench_identity_keygen,
@@ -134,5 +145,6 @@ criterion_group!(
     bench_identity_verify,
     bench_sealed_sender,
     bench_padding,
+    bench_safety_number,
 );
 criterion_main!(benches);