v1.72.0

Added

• Paywall.renderMode, .customScreenId and .customPayload — a paywall
  can now be a host-rendered custom screen instead of an authored element tree.
  renderMode: "custom" means the HOST draws it: customScreenId names a
  screen registered on PaywallHost (UI package), and customPayload is a map
  of slot key to per-platform store product id
  ({ monthly: { ios, android } }) — the one thing a native paywall cannot get
  from anywhere else, since the moment waterfall picked this variant and which
  products it offers is an authoring decision.

  All three are optional, and an absent renderMode reads as "elements".
  Not because the studio omits them — it always sends a value — but because a
  device on this SDK can be talking to an older get-paywalls that predates the
  fields, and on that pairing every existing paywall must behave exactly as
  before.

  A property of the PAYWALL rather than the moment, the same as billing: one
  moment audience can weight an element-tree variant against a native-screen
  variant and ramp the change as an A/B test.

• PaywallCustomPayload — exported so a host's custom screen can type the
  product map it receives without restating the shape.

• PresentErrorReason: "unknown-custom-screen" — a renderMode: "custom"
  paywall named a screen this host did not register (or named none at all), so
  nothing could be rendered and the Modal was never opened. Deliberately NOT
  folded into "parse-error": that one is a CMS data bug the studio author must
  fix, this is a wiring bug the app must fix. Hosts switching exhaustively on
  PresentErrorReason must widen that switch.

Changed

• No store products are resolved for a custom paywall.
  collectProductRefs does not walk customPayload, so such a paywall issues
  no store round-trip at all and its screen receives product ids rather than
  prices. A native paywall asks the store for its own display prices, which is
  precisely what it does not need the studio for.

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