Introduces llm.Parse(spec) backed by an extensible Registry that resolves
model strings like "openai/gpt-4o", aliases like "fast", and named targets
like "m5/qwen3:30b" (via LLM_M5 env var DSNs) into ready-to-use *Model
objects. Extension points: RegisterProvider, RegisterAlias, RegisterResolver.
Adds Foreman constructor and sentinel errors ErrAliasLoop, ErrUnknownProvider,
ErrInvalidDSN.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
