Add CodSpeed continuous performance testing

[codspeed-hq]

Summary

This PR integrates CodSpeed for continuous performance testing on the moonrepo/moon repository. CodSpeed provides accurate, reproducible performance measurements in CI, enabling detection of performance regressions on every pull request.

Changes

Dependency update -- Replaced criterion2 with codspeed-criterion-compat (v4.5.0) in:

• Cargo.toml (workspace dependency)
• crates/affected/Cargo.toml
• crates/workspace/Cargo.toml

The compatibility layer is a drop-in replacement for criterion, so no changes were needed in the benchmark source files (crates/affected/benches/affected_tracker.rs and crates/workspace/benches/workspace_graphs.rs). When not running under cargo codspeed, it behaves identically to standard criterion, so cargo bench continues to work as before.

CI workflow -- Added .github/workflows/codspeed.yml that:

• Triggers on pushes to master, pull requests (scoped to Rust-relevant paths), and workflow_dispatch (for CodSpeed backtest analysis)
• Uses OIDC authentication (id-token: write)
• Builds and runs benchmarks with cargo codspeed in simulation mode
• Uses CodSpeedHQ/action@v4

README -- Added the CodSpeed performance badge.

Next steps

1. Install the CodSpeed GitHub App on the moonrepo organization if not already done: https://github.com/apps/codspeed
2. Activate the repository on CodSpeed at https://codspeed.io
3. Once merged, CodSpeed will automatically run a backtest via workflow_dispatch to generate baseline data
4. Performance reports will appear as PR comments on subsequent pull requests

Existing benchmarks covered

• moon_affected -- Tracks affected projects/tasks (sync and async) at scales of 10, 100, and 1000 projects
• moon_workspace -- Workspace graph building (sync and async) at scales of 100 and 1000 projects

[codspeed-hq]

CodSpeed setup failed

The CodSpeed integration is fully implemented and committed locally (branch: codspeed-wizard-1776465467978), but could not be successfully deployed. PR #2493 exists but its CI workflow fails with OOM during the benchmark build step because the workspace's release profile (inherited by bench) uses LTO and codegen-units=1. The fix — overriding these via CARGO_PROFILE_BENCH_LTO=false and CARGO_PROFILE_BENCH_CODEGEN_UNITS=16 environment variables — is committed locally (commit cbbb464) but could not be pushed: the authentication token expired and the PR submission tool reached its maximum of 3 attempts. The workflow file and all code changes are correct and ready to be pushed once authentication is restored.