Implement SettlementBackendDriver interface + fake driver, per #6098's spec

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Problem

#6098 defines the settlement-backend interface at the spec level (event contract, balance query/decrement, refund/dispute hook points) but nothing has actually been implemented in code yet — there's no SettlementBackendDriver type or fake/in-memory driver the way control-plane/src/tenant-provisioning-driver.ts already has one for tenant provisioning.

Area

Platform / control-plane. Mirrors an existing, proven pattern in this same codebase.

Proposal

Implement SettlementBackendDriver (interface) + createFakeSettlementBackendDriver (in-memory implementation), directly mirroring control-plane/src/tenant-provisioning-driver.ts's shape: an interface capturing #6098's already-spec'd contract (payout-eligible event intake, balance query/decrement, refund/dispute hook), plus a fake driver with no real money movement, so #4792's ledger work and future settlement-scenario logic (#4791) can build and test against a stable interface regardless of which real backend (gittensor-owned or self-built) eventually lands behind it.

⚠️ Required pattern — read control-plane/src/tenant-provisioning-driver.ts in full before starting. The new interface + fake driver must mirror its exact shape (interface + createFake*Driver naming, in-memory Set/Map-backed state, idempotent methods). A PR with a differently-structured interface or driver factory does not satisfy this issue.

Deliverables

• SettlementBackendDriver interface covering Design spec: settlement-backend interface (gittensor-owned or self-built, pluggable either way) #6098's spec.
• createFakeSettlementBackendDriver, in-memory, no real funds/credentials.
• Unit tests exercising the fake driver's full interface surface.

Resources

• Design spec: settlement-backend interface (gittensor-owned or self-built, pluggable either way) #6098 (the spec this implements)
• control-plane/src/tenant-provisioning-driver.ts (the pattern to mirror)
• Rental ledger integration #4792 (rental ledger — the first real consumer)

Boundaries

Fake driver only — no real settlement backend, wallet/hotkey material, or reward figures in this issue or its implementation. Maintainer-only for now (this epic's alignment work isn't finalized), but flagged as a fast unlock candidate once the gittensor-alignment conversation lands, since a fake/interface-only driver touches no real funds and has a clear precedent to follow — closest thing in this milestone to a "safe to unleash" task once unlocked.

Contributor-eligible. Mirror tenant-provisioning-driver.ts's exact shape — fake driver only, no real settlement backend, no wallet/hotkey material, no reward figures.

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JSO-5369

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Unlocked to contributors (2026-07-20): reconsidered against this repo's own precedent — #4779/#4780/#4784-4788 (the generalized any-repo agent group) were all shipped by contributors under this exact pattern: a spec closes maintainer-only, then the interface/scaffolding implementation opens up once the spec is settled and doesn't itself require the undecided business call. #6098's interface spec is closed and already defines the contract; this issue builds a fake, in-memory driver against it — no real settlement backend, no funds, no credentials, mirrors the already-shipped control-plane/src/tenant-provisioning-driver.ts pattern directly. Doesn't presuppose which real backend (gittensor-owned registry-reweight, a funded pool, or something else) wins that's still an open cross-team question — that's the whole point of building the seam now.