Extract native share-file preparation from AndroidDocumentsPlugin

[Renakoni]

Summary

Eighth and final PR of the semantic-decomposition series: moves Markdown share preparation out of AndroidDocumentsPlugin.java into a focused SharePreparation helper. Behavior-preserving — file names, cache layout, FileProvider authority, ClipData shape, and log lines are unchanged.

Boundary

SharePreparation owns:

• the share-cache directory (shared-markdown under the app cache),
• temporary Markdown file writing behind the canonical-path containment guard,
• case-insensitive unique in-batch file naming and the suggested-name → safe-.md normalization,
• image attachment collection — the marktext-image://local/ link rewrite with its missing-file skip — and byte-for-byte attachment copying with unsafe-name rejection,
• ClipData construction over the prepared stream URIs.

ShareMarkdownPayload becomes a package-visible top-level class (same pattern as #91's value types). Two placement calls worth noting: normalizeSuggestedMarkdownName moves here as the front half of the TODO's "unique file naming" concern — the create/rename/shared-text call sites now reference it cross-feature rather than duplicating the rule; likewise hasSupportedImageExtension/normalizeImportedImageFileName move with their only remaining callers so the extension list cannot drift between share preparation and image import.

Per the TODO, the plugin keeps the final Capacitor and Android dispatch: both share plugin methods still validate through the codec, assemble the SEND/SEND_MULTIPLE intent with titles/flags, exclude the app from its own chooser, start the activity, and resolve the call.

Testing

• New SharePreparationTest (6 JUnit tests): suggested-name sanitization/extension handling, case-insensitive batch deduplication with extension-aware suffixes, unsafe imported-image-name rejection (traversal, wrong extension, null), the supported-extension rule, byte-for-byte attachment copying into a temp directory, and the path-traversal SecurityException guard. Native suite now 19/19 (MarkdownCodecTest 8 + IncomingIntentParserTest 5 + these 6), CI-gated since Extract native Markdown codec from AndroidDocumentsPlugin #91.
• Emulator smoke: long-press-selected two drafts and shared through the UI — shareMarkdownDocuments received both payloads and the new helper produced the share sheet end to end (Opened Android share sheet for 2 Markdown documents), exercising cache-dir creation, unique naming, cache writes, and ClipData on device.
• compileDebugJavaWithJavac + assembleDebug green; Java-only diff, TS contract untouched.