#3133 Model types: decide whether names like @/@x/y or a+b/c should be legal (type grammar)

Summary

Model types have no grammar. ModelType.create (src/domain/models/model_type.ts) rejects only input that is blank after normalization: lowercase, :: / . / whitespace folded to /, repeated slashes collapsed, leading and trailing / trimmed. Anything else is accepted, including @/@exp/probe, @@x/y and names containing + or other punctuation.

Should these be legal?

What exists today

• Published extension names must match ^@[a-z0-9_-]+/[a-z0-9_-]+(/[a-z0-9_-]+)*$ (extension_manifest.ts), and every model type in an extension must start with its @collective/ prefix.
• Local extension model types only need at least two segments (validateUserCollective in model_kind_adapter.ts); characters are not checked.
• All 9 built-in model types match ^@?[a-z0-9][a-z0-9_-]*(/[a-z0-9][a-z0-9_-]*)+$.

Why it matters

Several spellings can name, or almost name, the same type. Every comparison that makes a security or identity decision has to canonicalize defensively. swamp-club#3129 hit this: a leading-whitespace or doubled-@ spelling can differ from the stored type after a naive strip. A grammar would make one spelling canonical and reject the rest at the boundary.

Open questions

• Is any punctuation beyond - and _ intended in model type names? Is @ intended anywhere but the first character?
• Enforce in ModelType.create (affects stored definitions and local extensions across the CLI), or only for new definitions and extensions, with a warning for existing ones?
• Migration: how to find and handle existing local types and stored definitions that would no longer parse.

Until this is decided, #3129 canonicalizes rather than rejects, so no existing local type is blocked.