perf(extensions): bound and chunk RPC query result payloads

[ZhuchkaTriplesix]

Parent

Part of #414

Problem

Extension driver RPC uses newline-delimited JSON (LineSplitter + jsonDecode(line)). A db.query result can be one unbounded line → O(result) on the UI isolate before compute, plus another copy into the isolate.

Evidence

• lib/core/extensions/rpc/json_rpc_stdio_client.dart
• lib/core/extensions/sandbox/extension_driver_session.dart (query parse path)

Acceptance

• Max payload size enforced (fail closed with clear error)
• Prefer chunked / binary framing or streamed row batches for large results
• Parse off UI isolate before materializing large string grids
• Document protocol limits for driver authors

Suggested fix

Chunked framing or row-batch RPC; hard max line/payload size; mirror SQL workspace caps.

[ZhuchkaTriplesix]

Shipped on dev via #434 (Closes did not auto-close because PRs targeted dev).