ci(bench): set CodSpeed simulation mode (required in action v4)

[yisding]

Problem

The dependabot bump CodSpeedHQ/action v3 → v4 (#76) made the mode input required (no default). The deterministic instruction-count benchmarks job was the only one relying on the old implicit default, so it now fails on every run.

Fix

Set mode: simulation on that job — per the v4 action definition, simulation is the modern name for what v3 called instrumentation (now deprecated), i.e. the CPU-instruction-count measurement this job already intends. The two walltime jobs already pass mode: walltime, so they were unaffected.

       - name: Run CodSpeed benchmarks
         uses: CodSpeedHQ/action@v4
         with:
+          mode: simulation
           run: >-
             uv run --no-sync --python 3.13 pytest ...

One-line change (plus an explanatory comment). YAML validated; all three CodSpeed jobs now pass an explicit mode.

🤖 Generated with Claude Code

[codspeed]

Merging this PR will not alter performance

⚠️ Unknown Walltime execution environment detected

Using the Walltime instrument on standard Hosted Runners will lead to inconsistent data.

For the most accurate results, we recommend using CodSpeed Macro Runners: bare-metal machines fine-tuned for performance measurement consistency.

✅ 33 untouched benchmarks

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_{Comparing ci/codspeed-simulation-mode (d5803a7) with main (376d61f)}