feat(perry/system): #917 — shareText/shareUrl system share sheet

[proggeramlug]

Closes #917 (MVP — see follow-up matrix).

Summary

Two perry/system entry points wrapping the native share picker:

import { shareText, shareUrl } from "perry/system";
shareText("hello world");
shareUrl("https://example.com", "My link");

Use case: "share my invitation code" / "share this listing" flows where the OS share sheet hands the value to Messages / Mail / WhatsApp / etc. Wishare's invite-friends screen had to fall back to clipboardWrite + manual paste; native apps already expose this surface.

Cross-platform coverage

Platform	Status
macOS	Real impl — NSSharingServicePicker anchored to key window
iOS	Stub + #917 follow-up warning (UIActivityViewController)
tvOS	Stub (tvOS doesn't expose a programmatic share sheet)
watchOS	Stub (WatchKit doesn't expose a public API)
visionOS	Stub + #917 follow-up (window-anchored UIActivityViewController)
Android	Stub + #917 follow-up (Intent.ACTION_SEND chooser)
Windows	Stub + #917 follow-up (DataTransferManager.ShowShareUI)
Linux/GTK4	Stub + #917 follow-up (XDG share portal)
WASM	Stub via dispatch table fallback
HarmonyOS	Stub auto-generated by perry-runtime/build.rs

Every stub funnels through perry_runtime::stub_diag::perry_stub_warn, so first call per process per symbol prints a [perry] warning line naming the platform and the #917 tracker.

macOS implementation detail

NSSharingServicePicker initWithItems → showRelativeToRect:ofView:preferredEdge: anchored to the key window's content-view bounds (NSRectEdgeMinY so the popover renders above the view). shareText wraps the value as NSString; shareUrl wraps as NSURL so the picker offers Safari / Reading List / Bookmarks alongside Messages / Mail / Notes. Malformed URLs fall back to plain-text sharing.

The title argument is currently dropped on macOS (Cocoa's picker derives its label from the item type); kept in the signature for cross-platform symmetry with iOS/Android where title is meaningful.

Plumbing (mirrors #918's takeScreenshot pattern)

• API manifest, dispatch table, every per-platform UI crate, testkit feature matrix, HarmonyOS auto-stub via build.rs. WASM falls through to ui_method_to_runtime() so no parallel edit there.

Smoke test

Compiled App({body}) + shareText() + shareUrl() against release perry; symbols resolve end-to-end, link succeeds, binary writes cleanly.

What's deferred (#917 follow-ups)

• iOS / Android / Windows / Linux / visionOS native impls — issue maps each native API.
• shareItems({text?, url?, title?}) structured variant — dispatch is forward-compatible; a new MethodRow will land it without changing the two convenience entry points.

Notes

No Cargo.toml version bump, no CLAUDE.md touch, no CHANGELOG.md entry — maintainer folds those in at merge time.