feat(core-models): language-neutral canonical models with provenance

[rahlk]

Language-neutral canonical core models (Pydantic), e.g. under cldk/models/core/:

Symbol, Module, Type, Callable, Field, CallSite, Edge, Provenance(file, span) — every fact carries provenance.

Design constraints

• Typed per-language ext payload for language-specific fields (TS type aliases, Java annotations, ...) so the core stays small and closed.
• JSON-serializable with stable field names — this becomes the wire format for the query core (Composable, chainable query API: pa.callables().with_decorator(...).reachable_to(...).without_passing_through(...) #155), the backend schema contract, and cocoa-py.
• Existing per-language models are NOT removed; adapters map onto the core.

Depends on the URI RFC for identity fields.

Part of #194. Branch: feat/issue-<n>.