feat(benches): criterion benchmarks for signature verification paths

Adds criterion-driven benchmarks for the four signature-verification hot
paths: Ed25519 verify, DSSE envelope parse+verify, Merkle tree validation
(parameterised by leaf count), and cert-chain validation (parameterised
by chain length, matching MAX_CHAIN_DEPTH=8 from audit PR #98).

SLH-DSA bench deferred to issue #46. CI integration deferred to a
follow-up PR — this commit only lands the bench harness so baselines
can be captured.

Traceability comments at the top of the bench file link each group to
its CR-* artifact.

Implements: #89
Verifies: CR-1, CR-3, CR-7, CR-8

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: avrabe

Cargo.lock

@@ -109,6 +109,12 @@ optional = true
 [dev-dependencies]
 # WAT (WebAssembly Text) parser for tests
 wat = "1.221"
+# Criterion for signature-verification benchmarks (issue #89)
+criterion = { version = "0.5", features = ["html_reports"] }
+
+[[bench]]
+name = "verification_benchmarks"
+harness = false
 
 [features]
 default = ["software-keys", "sync"]