chore: benchmark suite for math-heavy lib functions

[IbrahimIjai]

Add Criterion benchmarks for: mul_div_wide, pow_factor, get_pool_value, get_position_fees, and swap_with_path (single and multi-hop). Run benchmarks in CI (report only, don't fail) to track performance regressions over time.

Acceptance criteria:

• cargo bench runs without errors
• Benchmark output committed to benches/
• CI runs benchmarks and uploads results as artifact

[Arome8240]

@Arome8240 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can I work this issue?

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Arome8240 to this issue.

[designsage8]

@designsage8 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Can i work on this ??

ℹ️ Repo Maintainers: To accept this application, review their application or assign @designsage8 to this issue.

[drips-wave]

Congratulations, @designsage8! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

🧑‍💻 @designsage8: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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