Publicize: public per-post filter for external/app publishers to set the share decision

### Summary

I maintain [Moment](https://github.com/jeffpaul/moment), a phone-first WordPress publishing app. Moments are standard `post`s, so Jetpack Social / Publicize shares them on publish. The gap: from Moment's own mobile UI there's no clean way to let a user choose, **per post**, whether a Moment is publicized, because Publicize's per-post connection controls live in the block editor pre-publish panel that a lightweight app bypasses.

Moment already offers an in-app, per-post toggle for plugins that expose a public control hook — for example Share on Mastodon's `share_on_mastodon_enabled` filter and Autoshare for Twitter's `autoshare_for_twitter_enabled_default` filter (both receive the post ID and let an external plugin decide, per post, whether that post is shared).

### Request

Is there a public, per-post filter an external plugin can hook to set Publicize's share decision programmatically (per post, receiving the post ID) — ideally without reconstructing the block-editor connection UI or writing the `_wpas_*` skip meta directly? If not, would you consider a documented extension point for headless/app publishers? Happy to discuss or open a PR. Thanks for Jetpack!

Start by reviewing Publicize's per-post connection controls and the existing `_wpas_*` skip-meta behavior, then compare them with the named `share_on_mastodon_enabled` and `autoshare_for_twitter_enabled_default` filters. Done means establishing whether a public per-post hook receives the post ID and, if not, defining and documenting an extension point for app publishers.

Written by the indexing model from the issue text.