feat: stable IDs in snapshot exports

[danmaps]

Closes #9.

What

• Adds id fields to exported JSON for:
  • maps.json (MapInfo.id)
  • layers.json (LayerInfo.id)
  • tables.json (TableInfo.id)
• IDs are deterministic UUIDv5-style GUIDs derived from a fixed namespace + a stable-ish key.

Stability guarantees (best-effort)

• For data-backed layers/tables, DataSourcePath is included in the key to keep IDs stable across display-name changes.
• For maps and group layers, renames can change the derived ID (we don’t currently have a true SDK GUID exposed in this exporter).

Notes

• This is purely additive schema: existing consumers can ignore id.
• Follow-up (optional): show IDs in arcgispro inspect output.