Update CodSpeed workflow to v4 with OIDC authentication

[codspeed-hq]

This PR updates the existing CodSpeed integration to use the latest best practices.

Changes

CodSpeed workflow (.github/workflows/codspeed.yml)

• Upgraded CodSpeedHQ/action from v3 to v4 -- the latest major version with improved features and stability.
• Switched from token-based auth to OIDC authentication -- removed the secrets.CODSPEED_TOKEN dependency and added id-token: write permission. OIDC is the recommended authentication method for CodSpeed, removing the need to manage repository secrets.
• Added explicit mode: simulation -- uses CodSpeed's CPU simulation instrument for consistent, hardware-agnostic measurements that are not affected by CI runner load.
• Added a workflow_dispatch comment explaining its purpose for CodSpeed backtesting.

README

• Added the CodSpeed performance badge at the top of the README.

Existing setup retained

The repository already had a well-configured benchmark setup:

• @codspeed/vitest-plugin v5.3.0 integrated in vitest.bench.config.ts
• Comprehensive benchmarks in bench/operations.bench.ts and bench/difference.bench.ts covering tree operations, search, chop, merge, set operations, and more.

No changes were needed to the benchmark files or plugin configuration.

Next steps

1. Install the CodSpeed GitHub App on this repository if not already done.
2. Once installed, trigger the workflow manually via workflow_dispatch to generate initial baseline data.
3. After that, CodSpeed will automatically run on every push to main and on pull requests, reporting performance changes inline.

---

^{Need help on this PR? Tag @codesmith with what you need.}

• Let Codesmith autofix CI failures and bot reviews

[codspeed-hq]

Merging this PR will not alter performance

✅ 25 untouched benchmarks

---

_{Comparing codspeed-wizard-1778638784986 (ea9c356) with master (95932f2)}