fix(machine): re-pair wipes the prior operator's config + watermarks (#70)

A new-seed re-pair UPSERTed the bunker binding but left fee_config, cassettes,
and the created_at replay watermarks intact. The watermarks are the trap: a new
backend whose first config event has a lower created_at than the old operator's
last event is silently dropped as a replay, so re-pairing a long-lived install
to a fresh backend appears to pair but never picks up new config.

Add resetForRepair() (main-process state-store): in one transaction it clears
fee_config and resets both replay watermarks to 0. Wired function → IPC
(state:reset-for-repair) → preload → renderer, and called from the re-pair branch
in signer-resolver, gated on an existing binding (re-pair only; a first pair has
nothing to reset). Deliberately preserves cassettes/cashbox/transactions — those
track PHYSICAL cash that survives an operator handover; a full wipe is the
factory-reset path.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/machine/electron/preload.ts

@@ -118,6 +118,7 @@ contextBridge.exposeInMainWorld('electronAPI', {
     ipcRenderer.invoke('state:save-bunker-binding', binding),
   clearBunkerBinding: (): Promise<void> => ipcRenderer.invoke('state:clear-bunker-binding'),
   resetBootstrapGate: (): Promise<void> => ipcRenderer.invoke('state:reset-bootstrap-gate'),
+  resetForRepair: (): Promise<void> => ipcRenderer.invoke('state:reset-for-repair'),
 
   // QR-pairing wizard (aiolabs/bitspire#52): persist a scanned spire-seed,
   // then relaunch so the normal boot flow pairs it.
@@ -239,6 +240,7 @@ declare global {
       saveBunkerBinding: (binding: BunkerBindingRecord) => Promise<void>
       clearBunkerBinding: () => Promise<void>
       resetBootstrapGate: () => Promise<void>
+      resetForRepair: () => Promise<void>
       saveSpireSeed: (seed: string) => Promise<void>
       relaunchApp: () => Promise<void>
       applyOperatorCassettesConfig: (