Implement verifier cost estimator for Halo2 circuits

[dkaidalov]

1. Build a tool that estimates the on-chain cost of running a generated Halo2 verifier, reporting operation counts (scalar and point arithmetic, hashes, transcript reads, pairings) and byte-size estimates for the proof and verification key.

2. Provide two modes: an exact computation that walks a fully extracted circuit representation, and a lower-bound estimate derived from circuit parameters alone (number of advice/fixed columns, lookups, public inputs, circuit degree) without needing a concrete circuit.

3. Surface the resulting cost figures in the existing examples so they can be inspected during development.