Add a lightweight, temporary trace logger to diagnose where the iOS
Share Extension stops executing during a failed cold-start share.
- ShareViewController: add appendTrace() helper that writes ISO8601-
prefixed lines to share-extension-trace.log in the App Group
container, ignoring failures (diagnostics only)
- Add trace entries across the share flow: viewDidLoad,
processAndOpenApp, processSharedImage, image attachment/load,
storeImageData, setSharedPhotoReadyFlag, openMainApp, completeRequest
- SharedImageUtility: add getShareExtensionTrace() (read-only) and
clearShareExtensionTrace() (deletes the trace file)
- SharedImagePlugin: expose getShareExtensionTrace() and
clearShareExtensionTrace() to JS
- definitions.ts / SharedImagePlugin.web.ts: add ShareExtensionTrace
type, method signatures, and web stubs
Share behavior is unchanged and Android is untouched. All additions are
marked with "TEMPORARY SHARE TARGET DIAGNOSTICS".
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