[Flash303]

Serve server-list prestats in the same binary format as GET /records/{id}, base64-encoded in a new stats_data field instead of JSON.

Backend:

• BiggerServerResponse.data: Option → stats_data: Option<Vec>, serialized as base64 (serialize_opt_base64)
• Reuses the existing RecordData::into_binary layout (u32 count + i64 base + N×u32 deltas + N×i32 values, LE)

Frontend:

• New shared decoder api/records-binary.ts (parseRecordBinary, base64ToArrayBuffer, decodeStatsBase64), now used by both fetchRecords and normalizeServerData
• normalizeServerData converts stats_data → data: ServerRecord[] and strips the base64 payload; legacy [dates, values] tuple still supported
• Values now read as i32 to match the backend layout
• 10 new tests (records-binary.test.ts): round-trip, empty/truncated buffers, invalid base64, legacy tuple
  Payload win: prestats no longer pay the JSON number-array overhead on /servers?include_stats=true.