Add draft improver core engine

[nonggde]

Closes #485

Summary

• add a folder-local deterministic Draft Improver core service
• expose a local API surface for improving single drafts, batches, validation, loading state, and error state
• add synthetic core fixtures plus a Node test covering request contracts, output changes, validation, and batch summaries
• document the input/output/loading/error contract in a new core contract note

Scope

• only touches tools/v2/individual/draft-improver/
• no app shell, routing, auth, wallet, Stellar, database, inbox, mail rendering, or design-system integration
• no live network calls, secrets, production data, LLM calls, persistence, or side effects

Validation

• node --test tools/v2/individual/draft-improver/tests/draft-improver-core.test.mjs
• npx --yes prettier --check on the new service, test, fixture, docs, and index files
• import smoke check for tools/v2/individual/draft-improver/index.mjs
• git diff --cached --check