Get Subgraph

Return subgraph for visualization — Spec 8 / S1.1, S1.4, S1.5.

Pagination contract:

• limit in [1, 500]; out-of-range values yield 400 (not 422) per AC-11 so clients get a human-readable reason instead of Pydantic’s validation schema.
• cursor is an opaque base64 token emitted by a prior call. A corrupted cursor yields 410 Gone per AC-12 — it cannot be silently reinterpreted as “first page”.
• Response always carries next_cursor. It is None when the returned page is the last one so clients can stop paging without a second round trip.