chore: adopt TypeScript 7 as baseline and document multi-version compatibility

[martyy-code]

Description

TypeScript 7 (GA July 2026) ships a native Go-based compiler with 8–12× faster builds, shared-memory multithreading, and --checkers N for configurable parallelization. We should:

1. Bump the repo's engines constraint to accept TypeScript 7
2. Update CI to run tests against TS 7 by default
3. Document the multi-version compatibility strategy in the README and the web app

Why

• TypeScript 7 is now GA and is the recommended version for new projects
• The 10× speed improvement makes deep type-checking in CI practically free
• Adopting TS 7 early positions the library as modern and fast-to-test

Acceptance criteria

• engines constraint updated to include >=7.0.0
• CI runs on TypeScript 7
• README documents the minimum supported TypeScript version
• Web app mentions TS 7 compatibility

Risks

• Low. TS 7 has no breaking changes for the type system — existing tests should pass unchanged.

Related

• ci: add TypeScript version matrix (5.x, 6, 7) for type tests #48 (ci: add TypeScript version matrix)
• https://devblogs.microsoft.com/typescript/announcing-typescript-7-0/