refactor(scannable): introduce Scannable hierarchy replacing Bill

[bmc08gt]

What

Replaces the single-variant Bill sealed interface with a Scannable ancestor that holds only the scannable code, and payment-bearing children under Scannable.Payable:

• Scannable — holds only data (the encoded KikCode bytes)
• Scannable.Payable — token/amount/didReceive/…, metadata, stamped()
  • Scannable.CashBill, Scannable.GoldBar
• Scannable.TipCard — directly under Scannable (non-payment)

Why

Bill conflated three things: the domain concept, the render choice (the renderAsBill flag), and a payment sub-kind. Hoisting a Scannable parent turns the render choice into a real type distinction and lets non-payment scannables (tip cards) reuse the bill pipeline later without dragging payment concerns along.

Changes

• forToken factory — the USDF→GoldBar rule (formerly the runtime renderAsBill flag) is now decided once at construction; renderAsBill is removed.
• stamped(code, nonce) replaces .copy(data =, nonce =) at the sealed-interface seam in presentBillToUser (a data-class copy isn't callable on a Payable reference).
• RenderedBill dispatches on type (is CashBill / is GoldBar) instead of the boolean flag.
• Migrated every consumer: scan / cash-link / currency-creator / customization delegates, AnimatedBill, and the three customization guards (narrowed to Scannable.CashBill).
• Existing tests migrated; new ScannableTest covers forToken routing + stamped type-preservation; the airdrop path now asserts awaitGrab is never called.
• Drive-by: a one-line correctness fix to the image-moderation error message in the same currency-creator file.

Testing

• ./gradlew :apps:flipcash:app:assembleDebug — BUILD SUCCESSFUL (standalone)
• :apps:flipcash:core and :apps:flipcash:shared:session unit tests pass