Bump Plonky3 and add prover cost analysis (#46)

* chore(deps): bump Plonky3 to e9d75614 and adapt to breaking API changes

- Implement `Dup` trait for `SymbolicExpression<F>` (new bound on `PrimeCharacteristicRing`)
- Rename `packed_linear_combination` to `batched_linear_combination` with updated signature
- Fix imports: add `Algebra` + `Dup`, remove unused `PackedField`

* docs: add prover cost analysis with per-phase breakdown and inline annotations

Document the prover's computational cost model in the module docstring,
covering all five phases: stage 1 commit, lookup trace construction,
stage 2 commit, quotient computation/commit, and FRI opening. Includes
notation, concrete formulas, and dominant-term analysis. Add inline cost
comments at each key step in prove_multiple_claims.

Author: arthurpaulino