Add proration mode for subscription upgrades / downgrades

[proggeramlug]

Today `js_play_billing_purchase(productId)` only supports buying a brand-new subscription — there's no way to upgrade an existing subscription to a higher tier or downgrade to a lower one. Apps that offer multiple subscription tiers will hit this immediately.

Play Billing API

`BillingFlowParams` has a `SubscriptionUpdateParams` builder that takes the current subscription's purchase token plus a `replacementMode` enum:

• `CHARGE_FULL_PRICE` — charge the new price, no proration
• `CHARGE_PRORATED_PRICE` — charge the difference for the rest of the period
• `WITHOUT_PRORATION` — effective on next renewal
• `WITH_TIME_PRORATION` — extend the period based on credit
• `DEFERRED` — change applies on next renewal, no charge now

Each mode has different UX implications. Apps need to expose the choice, or pick one based on policy.

Proposed API

Add a third optional parameter to `purchase`:

```typescript
export type ReplacementMode =
| "chargeFullPrice"
| "chargeProratedPrice"
| "withoutProration"
| "withTimeProration"
| "deferred";

export type PurchaseOptions = {
offerToken?: string;
/** Pass to upgrade / downgrade an existing subscription. */
replaceFor?: {
oldPurchaseToken: string;
replacementMode: ReplacementMode;
};
};

export declare function js_play_billing_purchase(
productId: string,
options?: PurchaseOptions,
): Promise;
```

(This obsoletes the simpler signature in #2 — they should land together. Or land #2 first as a milestone.)

Why deferred

Subscription upgrades have non-trivial UX implications (the user is changing their plan, not buying a new one). v0.1.x intentionally only covers the new-purchase case so apps can ship one-tier subscriptions. Multi-tier apps need this, but they're a smaller subset.

Work

• FFI declaration: pass options as a JSON string and parse on the Kotlin side (lets us add fields without touching the JNI signature) — or pick a flat `(productId, offerToken, oldToken, replacementMode)` shape and accept "" for absence. Decide which is cleaner.
• Rust: encode/forward the options object to Kotlin.
• Kotlin: parse, build `SubscriptionUpdateParams`, attach to `BillingFlowParams`.
• Document the upgrade flow + the proration semantics in README.
• Cross-platform note: StoreKit upgrades/downgrades work differently (Product.subscription.subscriptionGroupID + Transaction.upgrade); the cross-platform README example needs updating to show both flows side by side.