docs(book): add a simulator chapter (costs, synthetic data, state snapshots)

[ReverseZoom2151]

Adds a "The simulator" chapter to the mdBook documenting the sharpebench-sim engine features that had no prose docs, following up on the README update.

• Execution costs: fees, slippage, market impact, financing, liquidity caps, and the turnover (TRF) cost (trf_factor), with the determinism guarantees.
• Synthetic data: the baseline synthetic generator and the synthetic_parameterized variant (volatility multiplier + jump probability), including the exact-reproduction property when the knobs are at their defaults.
• State snapshots: clone_state / restore_state and EnvState for O(1) branch-and-restore over the same frozen data, with the RNG cursor in the snapshot keeping every branch deterministic.

Linked from SUMMARY.md between the Methodology section and "Submitting an agent". Docs-only.

[coderabbitai]

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ℹ️ Review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: b61d95bc-e0e4-400d-a044-80723af86425

📥 Commits

Reviewing files that changed from the base of the PR and between ca1c288 and e61a013.

📒 Files selected for processing (2)

• docs/book/src/SUMMARY.md
• docs/book/src/simulator.md

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/simulator-chapter

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