feat: add encode_members helper + fix README accuracy (#2, #3)

[peetzweg]

encode_members helper

Adds a built-in WASM function:

import { encode_members } from 'verifiablejs/nodejs'
const encodedMembers = encode_members(memberKeys) // Uint8Array[] of 32-byte keys

It SCALE-encodes an array of 32-byte member public keys into the Vec<Member> shape that every ring function (one_shot, validate, is_valid, batch_validate, members_root, members_intermediate) expects for its members parameter. Previously every caller had to hand-roll the SCALE Vec<[u8; 32]> encoding or reach for @polkadot-api/substrate-bindings / @polkadot/util.

• Decodes each element as a Member (not just byte concatenation), so the output round-trips with the existing internal decode and each key is validated.
• Explicit 32-byte length guard → clear error instead of silent truncation.
• Adds a round-trip test and a wrong-length negative test.

README accuracy

The published package README was stale and never mentioned the pre-flight validator the feedback specifically asks for:

• Fixed Quick Start: domain_size 11/12/16 → ring_exponent 9/10/14 (the values the API actually accepts; one_shot(11, …) would have thrown).
• Surfaced validate_with_commitment in both READMEs as the recommended local pre-flight before an on-chain submission — validate against the 768-byte ring root the chain exposes.
• Both READMEs now point at encode_members for building the members argument, keeping the hand-rolled encoding only as a no-WASM fallback.

Out of scope (follow-up PR)

The root README also documents create_multi_context / validate_multi_context / is_valid_multi_context / MultiContextResult, which do not exist in lib.rs. Removing those phantom docs is handled in a separate PR.

Verification

• cargo check --target wasm32-unknown-unknown and cargo check --tests --target wasm32-unknown-unknown both pass.
• dprint check clean on changed files.
• ⚠️ The wasm-bindgen tests were not executed — wasm-pack is not installed locally, so pnpm test couldn't run. The new tests type-check but should be run in CI before merge.