feat: add /positions/reference endpoint

[jonathanchw]

Summary

• Adds GET /positions/reference endpoint returning the best reference position per collateral
• Adds isChallenged to PositionQuery type and Ponder GraphQL query
• Filters: not closed, not denied, not challenged, principal > 0, not in cooldown, not expired
• Returns highest priced valid position per collateral for cooldown-free price increases

Why

Provides a valid reference position so bapp can call adjustPriceWithReference / adjustWithReference
without reverting with InvalidPriceReference.
bapp receives the map, looks up map[position.collateral], checks ref.price >= newPrice, and uses the reference if valid.

Smart contract alignment

The API filters mirror the 9 checks in Position.sol _isValidPriceReference():

#	Contract check	API filter
1	Same hub	Implicit (all positions from same Ponder instance)
2	Reference ≠ this position	Handled by bapp at call time
3	Same collateral	API returns one ref per collateral key
4	Not in cooldown (block.timestamp > ref.cooldown())	cooldown < now ✓
5	Not expired (block.timestamp < ref.expiration())	expiration > now ✓
6	Not challenged (ref.challengedAmount() == 0)	isChallenged == false ✓
7	Not closed (!ref.isClosed())	closed == false ✓
8	newPrice <= ref.price()	Checked by bapp/contract at call time
9	ref.principal() > 0	principal > 0 ✓

Response shape

{
  num: number;            // number of collaterals
  collaterals: Address[]; // list of collateral addresses
  map: {
    [collateral: Address]: PositionQuery; // full PositionQuery per collateral
  }
}

Test

curl -s http://localhost:3001/positions/reference | jq

Returns the highest-priced valid reference position per collateral. Verified locally against dev Ponder (dev.ponder.juicedollar.com).

Depends on

JuiceDollar/ponder#19 (isChallenged field on PositionV2) — merged ✓