perry/system: native iOS share sheet (UIActivityViewController) #917

Request: Expose a system-share API from perry/system that wraps UIActivityViewController on iOS (with parallel implementations on macOS/Android where applicable).

Proposed TS signature:

```ts
// perry/system
export function shareText(text: string, title?: string): void;
export function shareUrl(url: string, title?: string): void;
// Optional: structured shape with multiple items
export function shareItems(items: { text?: string; url?: string; title?: string }): void;
```

Why it's needed: apps frequently want to "share my invitation code" / "share this listing" via the OS share sheet so the user can pick Messages / Mail / WhatsApp / etc. Today this isn't possible — Wishare's invite-friends screen has to fall back to `clipboardWrite` and a manual paste, which is a worse UX than every native social/marketplace app the user is comparing against.

Platform mapping:

Context: Audited docs.perryts.com — no `shareSheet`, `shareText`, or `UIActivityViewController` references. `openURL`, `clipboardRead/Write`, `imagePickerPick` are documented, but a one-shot share to the OS picker is the obvious next neighbour.

Happy to take a stab at the iOS implementation if useful; the perry-ui-ios crate already has objc2 wired in.