feat(machine): seed + bunker-binding IPC bridge

get-atm-secrets now returns { spireSeed, bunkerBinding } instead of the raw
nsec (one-shot semantics kept). Adds IPC handlers + preload bindings for
saveBunkerBinding / clearBunkerBinding / resetBootstrapGate so the renderer
can persist a pairing and re-arm the cassette-state hello on re-pair (#56).
resetBootstrapGate added to state-store. Types mirrored in electron.d.ts.

Part of Phase C, aiolabs/bitspire#52.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/machine/electron/preload.ts

@@ -42,13 +42,25 @@ export interface BrandingConfig {
   logoDarkDataUrl: string | null
 }
 
+/**
+ * Persisted NIP-46 bunker binding (mirror of state-store's StoredBunkerBinding).
+ */
+export interface BunkerBindingRecord {
+  clientSecretHex: string
+  spirePubkey: string
+  bunkerUrl: string
+  seedFingerprint: string
+  pairedAt: number
+}
+
 /**
  * ATM secrets — returned once by getAtmSecrets(), then empty on subsequent calls.
+ * The spire pairing seed (carries the one-shot connect token) plus the persisted
+ * bunker binding; the renderer resolves these into a signer.
  */
 export interface AtmSecrets {
-  atmPrivateKey: string
-  /** Legacy LP admin token — retained until 3d removes the LP backend. */
-  adminToken?: string
+  spireSeed: string
+  bunkerBinding: BunkerBindingRecord | null
 }
 
 // Expose protected methods to renderer
@@ -100,6 +112,13 @@ contextBridge.exposeInMainWorld('electronAPI', {
     ipcRenderer.invoke('state:get-bootstrap-published-at'),
   markBootstrapPublished: (unixTimestamp: number): Promise<void> =>
     ipcRenderer.invoke('state:mark-bootstrap-published', unixTimestamp),
+
+  // Bunker binding persistence (aiolabs/bitspire#52)
+  saveBunkerBinding: (binding: BunkerBindingRecord): Promise<void> =>
+    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'),
+
   applyOperatorCassettesConfig: (
     payload: {
       positions: Record<string, { denomination: number; count: number }>
@@ -212,6 +231,9 @@ declare global {
       getLastKnownConfigCreatedAt: () => Promise<number>
       getBootstrapPublishedAt: () => Promise<number | null>
       markBootstrapPublished: (unixTimestamp: number) => Promise<void>
+      saveBunkerBinding: (binding: BunkerBindingRecord) => Promise<void>
+      clearBunkerBinding: () => Promise<void>
+      resetBootstrapGate: () => Promise<void>
       applyOperatorCassettesConfig: (
         payload: { positions: Record<string, { denomination: number; count: number }> },
         eventCreatedAt: number