[SPIKE] benchmark framework choice

[kirich1409]

Context

Evaluate XCTest performance measurements vs swift-benchmark. Output ADR in docs/adr/0002-benchmarks.md. New dep requires approval.

Acceptance criteria

• ADR written with decision.
• Dep approval recorded if needed.

Files to touch (estimate)

best-effort estimate; agent refines during implementation

Implementation notes

n/a

Dependencies

None.

Safety checklist

See .github/ISSUE_TEMPLATE/task.yml — tick applicable invariants in the issue if any destructive path is touched.

Definition of Done

• Acceptance criteria satisfied and verifiable
• Tests added or updated
• No build / lint / test regressions
• Self-review against research-report invariants
• Code review approved