mcp: intern element handles instead of minting one per mention

`element_to_handle` inserted a new arena slot on every call, so an element
mentioned in two replies received two handles. Repeated queries therefore
consumed the 10 000 slot budget for elements that were already tracked, and
the LRU evicted handles the client was still holding. Driving an editor with
a few hundred cards on screen minted roughly 8 000 handles for the same 400
elements.

Look the element up first and reuse its slot. `ElementHandle` compares by
pointer and doesn't hash, so the lookup buckets candidates by the item index
and element index that equal elements share, then compares within the bucket.
Entries whose slot was evicted, or whose element is gone, are dropped as they
are encountered, so eviction and destruction need no extra bookkeeping.

Handles now stay stable for as long as the element lives, which the server's
instructions state so a client knows it can hold on to them.

ChangeLog: The MCP server now returns the same handle for an element across queries, instead of a new one each time.