rfc: cldk:// symbol URI scheme (SCIP/LSIF-informed)

[rahlk]

Design RFC for the cldk:// entity URI scheme. Written ADR, no code — everything else in Epic A builds on it.

Must cover

• Grammar: cldk://<lang>/<module-or-package>#<symbol>(<params>) — escaping rules, overload disambiguation, nested and anonymous symbols (TS synthesized callables, Python lambdas/closures, Java anonymous classes).
• Stability guarantees: what survives re-analysis; what survives refactors.
• Language registry: adding language #12 is a table entry, not a redesign.
• Versioning of the scheme itself.
• Explicit survey of prior art — SCIP symbol monikers, LSIF — and where/why we deviate.

Deliverable: an ADR/RFC document under docs/, reviewed and merged.

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