Add email ownership core engine

[nonggde]

Closes #435

Summary

• add a folder-local Email Ownership Tracker core engine for claim, release, transfer, ledger derivation, conflict tracking, defensive copies, and UI-ready state mapping
• add deterministic synthetic ownership event fixtures covering active owners, transfers, releases, conflicts, and invalid input cases
• document the core API inputs, outputs, loading/empty/ready/error states, behavior guarantees, and future integration boundaries

Scope

• only touches tools/v1/team/email-ownership-tracker/
• no app shell, dashboard, routing, inbox architecture, mail rendering, auth, wallet, Stellar, database, notification, provider, analytics, or design-system integration
• no production data, live network calls, server persistence, secrets, webhooks, background jobs, mailbox mutation, ownership writes, or paid services
• avoids the old closed [V1][team] Email Ownership Tracker - Core feature engine #435 attempt paths: service.ts, types.ts, index.ts, tests/service.test.ts, README/specs, and fixtures/ownership-cases.ts

Validation

• node --test tools/v1/team/email-ownership-tracker/tests/ownership-engine.test.mjs
• npx --yes prettier --check tools/v1/team/email-ownership-tracker/core/ownership-engine.mjs tools/v1/team/email-ownership-tracker/fixtures/ownership-engine-cases.json tools/v1/team/email-ownership-tracker/docs/CORE_API.md tools/v1/team/email-ownership-tracker/tests/ownership-engine.test.mjs
• git diff --cached --check
• ASCII scan over changed Email Ownership Tracker core files

Notes

• this is an isolated core engine only; future UI or integration issues can call it through folder-local adapters before any product data or mailbox writes are introduced