Bump verifiable crate to rev 19b03de

[peetzweg]

Summary

Upgrades the verifiable crate dependency from rev f65b39d to rev 19b03de (upstream mainline, merge of paritytech/verifiable#59).

No JS API surface change. Proofs, signatures, ring roots, and member encodings remain wire-compatible with the previous rev — all existing cross-validation tests (JS vs Rust proof equality, commitment round-trips) pass unchanged.

What this pulls in from upstream

The previous pin matched the no-point-validation state; this moves onto mainline and picks up the hardening work from paritytech/verifiable#56–#59:

• Curve-point validation on decode re-enabled — Member, MembersCommitment (ring root), MembersSet, and StaticChunk bytes are validated (on-curve + correct subgroup) when decoded, so malformed input fails cleanly at decode instead of risking panics in downstream crypto.
• Identity point rejected as a member (Reject identity point in member validation and construction verifiable#57) — previously the neutral element passed is_member_valid but made commitment construction panic (a wasm trap). Both paths now reject it with a proper error. A regression test covering is_member_valid and members_root is added in this PR.
• Canonical KZG verifier-key pinning (Pin canonical KZG verifier key in member validation verifiable#58) — validate / validate_with_commitment / is_valid / batch_validate reject a ring root whose embedded trusted-setup key is not the canonical Bandersnatch one, closing a membership-forgery vector for attacker-supplied commitments.
• Static verifier/prover caches (Refactory verifiable#59) — ring-context parameters are no longer recomputed on every call, speeding up repeated verification within a session.
• secret-split bundled into std — side-channel resistant secret scalar multiplication now applies to the wasm prover paths (one_shot, sign). Uses the OS RNG via getrandom, which works under wasm through the already-enabled getrandom/js feature.
• Transitive ark-vrf bump 0.5.0 → 0.5.1.

Changes

• packages/verifiablejs/Cargo.toml / Cargo.lock: rev bump (+ ark-vrf 0.5.1)
• packages/verifiablejs/src/lib.rs: new test_identity_point_member_rejected regression test
• .changeset/upgrade-verifiable-19b03de.md: changeset (minor) for the next release

Testing

• cargo check --target wasm32-unknown-unknown clean
• pnpm build (both bundler and nodejs wasm-pack targets) clean
• pnpm test: all 26 wasm tests pass (25 existing + 1 new regression test)