give a lease-extend joiner its own shortfall

[bpapillon]

Extend is single-flight per slot, so a check that arrives while an extend is in flight joins it and gets whatever that extend asked for. A background extend for the default tranche running when a check needs more than the tranche left the check failing with insufficient lease balance while the server still had the credits.

Ports the shape from schematic-node #187. The flight records the additional amount its wire call asked for, sized one level above the wire call so the registered figure and the request body match. A joiner whose shortfall exceeds the flight's ask waits the flight out, re-reads the slot, and issues exactly one follow-up extend for the difference. A follow-up never chains, so a server that cannot cover the request does not spin, and the flight cleanup is identity-guarded so it cannot evict a follow-up registered under the same key. Watermark joiners and any check the tranche covers still share one wire call.

SPEC.md carries the same joiner rule as Node. Tests cover the follow-up sizing, the no-chain rule, shared wire calls for in-tranche joiners, the cleanup guard, and the end-to-end check that now reserves after one follow-up.