Export: stable IDs for maps/layers in snapshot

[danmaps]

Why

Downstream packs (smells/approach/automation feasibility) need stable identifiers to avoid name collisions and to diff reliably.

Proposal

Add stable IDs to snapshot/context output:

• Map: id
• Layer: id
• Table: id (if applicable)

Acceptance Criteria

• IDs are stable across exports for the same underlying object when possible.
• IDs are present in the flat files under .arcgispro/context/.
• CLI inspect displays IDs optionally (or at least they exist in JSON).

Notes

If ArcGIS Pro SDK exposes a GUID/item ID, prefer that. Otherwise derive an ID deterministically from (project path + item URI + map + layer path) and document stability guarantees.