feat(machine): open a verified Bolt Card session at tap-to-enter

Entry now spends the tap's single-use SUN once, on the card server's new
/session endpoint (aiolabs/boltcards feat/card-session-endpoint), instead
of parsing the lnurlw locally and deferring every check to Complete. The
server proves a genuine, non-replayed card and returns the wallet balance
plus the hit-keyed LUD-03 withdraw and LUD-06 pay second steps — the same
single-use bearer /scan and /pay hand out — so Complete still needs no
second tap and the ATM holds no p/c for the visit.

- electron/boltcard-session.ts: /scan → /session URL derivation, response
  parsing, 404 → 'card server does not support sessions'.
- lnurl-withdraw / lnurl-pay: the second steps are now callable on their
  own (executeWithdrawCallback, resolveInvoiceFromPayStep); the tap paths
  are unchanged and reuse them.
- IPC: lnurl:open-card-session, lnurl:withdraw-session, lnurl:pay-session.
- store: handleBoltCardEntry opens the session then authorizes the
  server-returned external_id; the payment handlers take a source (raw
  tap or session); a withheld withdraw step declines with the server's
  reason. The boltcard AccessScan no longer carries the lnurlw.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

apps/machine/electron/lnurl-pay.ts

@@ -59,6 +59,17 @@ interface CardPayTarget {
   lnurl?: string
 }
 
+/**
+ * The LUD-06 second step on its own: what a `payRequest` (or a Bolt Card
+ * session, see boltcard-session.ts) hands us to fetch an invoice.
+ */
+export interface PayStep {
+  callback: string
+  minSendable?: number
+  maxSendable?: number
+  metadata?: string
+}
+
 /** LUD-06 payRequest (subset) + error shape. */
 interface PayRequest {
   tag?: string
@@ -163,17 +174,18 @@ async function toPayRequest(
 }
 
 async function requestInvoice(
-  pr: PayRequest,
+  pr: PayStep,
   amountMsat: number,
   ctx: Ctx
 ): Promise<ResolveCardInvoiceResult> {
+  if (!(amountMsat > 0)) return { ok: false, reason: 'no amount to send' }
   if (typeof pr.minSendable === 'number' && amountMsat < pr.minSendable) {
     return { ok: false, reason: 'amount is below the card wallet minimum' }
   }
   if (typeof pr.maxSendable === 'number' && amountMsat > pr.maxSendable) {
     return { ok: false, reason: 'amount is above the card wallet maximum' }
   }
-  const cbUrl = appendQuery(pr.callback!, { amount: String(amountMsat) })
+  const cbUrl = appendQuery(pr.callback, { amount: String(amountMsat) })
   let vals: PayValues
   try {
     const res = await ctx.doFetch(cbUrl, { signal: AbortSignal.timeout(ctx.timeoutMs) })
@@ -222,5 +234,19 @@ export async function resolveCardInvoice(
   if (!pr.ok) return pr
 
   // 3) Ask for an invoice for the payout amount.
-  return requestInvoice(pr.payRequest, amountMsat, ctx)
+  return requestInvoice({ ...pr.payRequest, callback: pr.payRequest.callback! }, amountMsat, ctx)
+}
+
+/**
+ * The LUD-06 second step alone: fetch a BOLT11 for `amountMsat` from an
+ * already-obtained pay step (from a Bolt Card session opened at tap-to-enter).
+ * Never throws — every failure returns `{ ok: false, reason }`.
+ */
+export async function resolveInvoiceFromPayStep(
+  step: PayStep,
+  amountMsat: number,
+  opts: ResolveCardInvoiceOptions = {}
+): Promise<ResolveCardInvoiceResult> {
+  const ctx: Ctx = { doFetch: opts.fetchImpl ?? fetch, timeoutMs: opts.timeoutMs ?? 15_000 }
+  return requestInvoice(step, amountMsat, ctx)
 }