feat(bidding): add pending-bid ledger

[Farhad-Shabani]

Why

There's a blind spot between "we bid" and "we won": the auction takes ~15s to settle, but the bidder rebuilds its committed load every 3s from assigned requests only. So for a few passes it completely forgets what it just promised and happily bids again. We hit this for real on sepolia — threw 8 requests at it, and the tail ones got admitted way past capacity and blew their deadlines.

How

The bidder now keeps a little ledger of its outstanding promises (PendingBids in spn-bidding, right next to EstimateCache which works the same way):

• When admission says yes, the request goes into the ledger with its expected gas, right before the bid goes out.
• At the start of every pass, the ledger gets reconciled with reality: if the request shows up as assigned, we won and it's counted normally, so the entry drops. If it's biddable again, our bid never stuck — drop it and re-decide. If it expired, we lost the auction. Whatever's left still counts as committed load.
• Expiry is per entry: min_auction_period plus 15s of slack for the auctioneer tick and our next poll, capped at 60s. The slack errs on the late side on purpose — expiring early would double-promise an auction we actually won, which is the exact bug we're fixing. Worst case, a lost auction holds some headroom for one auction cycle.