ci: add bench-regression workflow for crypto_primitives_bench

[moudyellaz]

🎯 Purpose

Add a bench-regression GitHub workflow that uses boa-dev/criterion-compare-action to gate crypto_primitives_bench against the base branch on every PR that touches the watched paths. Follow-up to #488 (the criterion migration); deferred to this PR because the action needs the criterion bench to exist on both head and base.

⚙️ Approach

Workflow triggers on PRs touching tools/crypto_primitives_bench/**, key_protocol/**, nssa/core/**, or the workflow file itself. One job runs the action against the PR base, with cwd: tools/crypto_primitives_bench and benchName: primitives. The action posts the comparison as a PR comment.

• Add .github/workflows/bench-regression.yml.

🧪 How to Test

Open this PR; the workflow self-triggers because it touches the watched path (the workflow file itself). The first run on this PR will compare against main and post a no-change diff (since this PR doesn't touch bench code).

🔗 Dependencies

#488 merged. Main now has tools/crypto_primitives_bench/benches/primitives.rs for the action to compare against.

🔜 Future Work

None.

📋 PR Completion Checklist

• Complete PR description
• Implement the core functionality
• Add/update tests
• Add/update documentation and inline comments

[github-actions]

Benchmark for 58134a0

Click to view benchmark

Test	Base	PR	%
encryption/decrypt	634.9±1.70ns	634.0±2.32ns	-0.14%
encryption/encrypt	866.3±7.75ns	818.4±6.83ns	-5.53%
keychain/new_mnemonic	4.2±0.03ms	4.1±0.02ms	-2.38%
keychain/new_os_random	4.1±0.02ms	4.1±0.04ms	0.00%
shared_secret_key/sender_dh	135.1±0.50µs	136.0±4.11µs	+0.67%

[Pravdyvy]

lgtm