feat: bundle openai as an internal dependency, move to internal self types

[Abhinavexist]

openai is no longer a dependency of interfaze. Its runtime is bundled into the published package, and the SDK now owns its public TypeScript types instead of re-exporting openai's. Also adds first-class chat.completions.parse() for zod-validated structured output.

What changed

• Bundle: tsup noExternal: [/^openai(\/|$)/]; openai moved from dependencies → devDependencies, pinned exact 6.47.0.
• Types: new src/_compat.ts — the owned surface; every src/* imports its types from ./_compat.js, not openai.
• Structured output: interfaze.chat.completions.parse(...) + re-exported zodResponseFormat; returns a typed message.parsed (ParsedChatCompletion<T>).
• Visibility: internal names carry no openai (_compat.ts, Client); sourcesContent: false keeps openai source out of the published source maps.
• Docs: README structured-output leads with parse; Errors section notes "import error classes/types from interfaze, not openai."