feat: eliminate `this: any` in muya render system with StateRenderContext

Introduce StateRenderContext interface (renderContext.ts) capturing all
properties and methods available on StateRender mixin targets. Replace
`this: any` → `this: StateRenderContext` across 30+ render functions
(renderBlock, renderContainerBlock, renderLeafBlock, and all renderInlines).

noExplicitAny in muya/lib: 35 → 8 (remaining 8 are justified third-party
library exceptions). @ts-expect-error: 242 → 236.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>