v0.12.300 — A cached payment is reused only when it fits this request

A cached payment is reused only when it fits this request

To skip the 402 round trip, ClawRouter re-signs the last payment terms it saw
for a model ("pre-auth"). It reused them whenever they covered the new request,
including a much SMALLER one, and an exact payment settles the whole signed
amount. So a small call made after a large one paid the large call's price: in
the gateway's records for October 21–24, one $0.001 call paid $0.0098.

A cached payment is now reused only when it covers the request and exceeds its
estimate by at most 3%. Otherwise the request takes the normal 402 flow and pays
its own quote. The gateway now refuses a payment signed more than 3% over the
price (PAYMENT_OVERSIGNED, nothing charged), so older ClawRouter versions are
no longer overcharged either; they just spend an extra round trip.