build: upgrade Soroban dependencies to 27

[kalepail]

Summary

• upgrade the workspace from soroban-sdk 26.0.1 to 27.0.0
• upgrade the Tornado example from soroban-poseidon 26.0.0 to 27.0.0
• regenerate the workspace lockfile

This is a draft for the focused-mainline option discussed in #39. It intentionally does not duplicate the verifier hardening and Protocol-27 workflow changes in #38; maintainers can instead request that this dependency bump move onto that branch.

Validation

• cargo check -p ultrahonk_soroban_verifier --all-targets: passed
• cargo clippy -p ultrahonk_soroban_verifier --all-targets -- -D warnings: passed
• verifier unit tests: 12/13 passed; the remaining test_transcript_determinism requires the generated circuits/simple_circuit/target/proof, which is absent from a clean clone per the documented fixture workflow
• dependency inspection: the verifier resolves a single Soroban SDK/env stack at 27.0.0
• downstream real-fixture GameVerifier suite on SDK 27: 18/18 passed
• downstream Protocol-27 GameSession with a Wasm-registered real verifier: 18/18 passed, including valid-proof acceptance and mutated-proof exact rollback

Related dependency issue

A completely fresh SDK-27 lockfile may select incompatible ed25519-dalek 3.0.0 through soroban-env-host. That independent resolver issue is tracked in stellar/rs-soroban-env#1705 with a fix proposed in stellar/rs-soroban-env#1706. This branch's regenerated lockfile resolves the compatible 2.x line.

Refs #39.