A computed TypeScript field without an initializer (`[INTERNALS]: T`)
was refused even with a stable Symbol()/Symbol.for() or literal key, which
contradicts the documented rule that TypeScript fields need an initializer
or an unconditional constructor assignment.
Such a field now takes its declared type and the identifier fields'
definite-assignment rule: an unconditional top-level `this[key] = ...` in
the constructor, otherwise the checked deferred-initialization slot. Types
that admit undefined (optional fields) start undefined.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
