feat(ramp): add Headless Host + quote-first startHeadlessBuy (Phase 5)

Introduces the Headless Host screen as the stable stack base for the
Unified Buy v2 headless flow and switches `startHeadlessBuy` to a
quote-first signature so external consumers drive the flow without
seeding the RampsController.

- New `Routes.RAMP.HEADLESS_HOST` registered inside the Unified Buy v2
  inner stack (`TokenListRoutes`) so `useTransakRouting` resets land on
  the Host instead of BuildQuote.
- `useHeadlessBuy.startHeadlessBuy({ quote, ... })` derives the
  `ContinueWithQuoteContext` from the supplied quote, creates a session
  in the registry, and navigates into the v2 stack via the proper
  nested-screen descriptor (TOKEN_SELECTION → MainRoutes → HEADLESS_HOST).
- Single-active-session policy: starting a new session
  auto-`closeSession`s the previous one with `consumer_cancelled`.
- `HeadlessHost` orchestrates: on focus it picks up the session, calls
  `continueWithQuote` exactly once (guarded by `hasContinuedRef` +
  `session.status` so the OTP/auth loop's re-focuses don't re-trigger),
  and surfaces `nativeFlowError` from `OtpCode` as
  `onError({ code: 'AUTH_FAILED' })`.
- Native flow screens (`EnterEmail`, `OtpCode`, `VerifyIdentity`) now
  thread `headlessSessionId` so `useTransakRouting` can be parameterized
  with `baseRoute = HEADLESS_HOST`.
- Playground UI: per-quote "Start headless buy" buttons (standalone
  start button removed); tapping replaces any active session.
- BuildQuote `headlessSessionId` param marked `@deprecated` — the
  headless flow no longer routes through BuildQuote.

Tests: 64 passing across `useHeadlessBuy`, `useContinueWithQuote`,
`useTransakRouting`, `HeadlessHost`, and `HeadlessPlayground`.

Author: wachunei

app/components/UI/Ramp/Views/BuildQuote/BuildQuote.tsx

@@ -97,14 +97,13 @@ export interface BuildQuoteParams {
   /** Pre-fill the amount input (e.g. when restoring state after a navigation reset). */
   amount?: number;
   /**
-   * Active headless buy session id, if the screen was opened via
-   * `useHeadlessBuy().startHeadlessBuy(...)`. Threaded through navigation so
-   * downstream routing helpers can look up the session in
-   * `sessionRegistry` and fire the consumer's lifecycle callbacks instead of
-   * navigating to the order-processing screen.
+   * Legacy param from Phase 3. The headless flow now navigates straight
+   * to `Routes.RAMP.HEADLESS_HOST` and never lands on BuildQuote, so the
+   * field is unused. Kept as `optional` for backward compatibility with
+   * any in-flight deeplinks; safe to remove once we're sure no callers
+   * pass it.
    *
-   * Phase 3 only plumbs this param — the screen itself does not branch on
-   * it yet.
+   * @deprecated Use `Routes.RAMP.HEADLESS_HOST` instead.
    */
   headlessSessionId?: string;
 }

app/components/UI/Ramp/Views/HeadlessHost/HeadlessHost.styles.ts

@@ -0,0 +1,30 @@
+import { StyleSheet } from 'react-native';
+import { Theme } from '../../../../../util/theme/models';
+
+const styleSheet = (params: { theme: Theme }) => {
+  const { theme } = params;
+  return StyleSheet.create({
+    container: {
+      flex: 1,
+      backgroundColor: theme.colors.background.default,
+    },
+    body: {
+      flex: 1,
+      alignItems: 'center',
+      justifyContent: 'center',
+      paddingHorizontal: 24,
+      gap: 16,
+    },
+    spinner: {
+      marginBottom: 8,
+    },
+    text: {
+      textAlign: 'center',
+    },
+    cancelRow: {
+      paddingTop: 12,
+    },
+  });
+};
+
+export default styleSheet;