Add Post_Types projector; pivot onboarding SDD to direct AP option writes

[kraftbj]

Fixes DOTCOM-16875

Summary

Two linked changes around the "who owns the AP option" question:

1. New Automattic\Fosse\Post_Types projector (src/class-post-types.php) reads AP's stored activitypub_support_post_types and feeds it into Atmosphere's atmosphere_syncable_post_types filter. A user ticks a post type once in AP's settings; it federates to Mastodon and Bluesky. One-way, no FOSSE-owned option.

2. Onboarding SDD amendment (sdd/onboarding-setup-ux/). The merged spec (PR SDD: onboarding-setup-ux (requirements + spec + plan) #25) planned fosse_ap_actor_mode / fosse_ap_support_post_types options projected into AP via pre_option_* filters. Superseded by the direct-write approach: FOSSE's Setup page writes straight to activitypub_actor_mode / activitypub_support_post_types. AP's admin screen keeps editing the same keys — single source of truth, no divergence.

Why the pivot

Original plan for DOTCOM-16875 was a FOSSE-owned fosse_syncable_post_types option projected into both AP and AT via filters, symmetric with Object_Type. Landed Automattic/wordpress-activitypub#3218 to add a dedicated AP filter for that.

AP maintainer pushed back: the stock option_activitypub_support_post_types filter already exists, add_post_type_support() covers adds, so a new filter isn't warranted. Fair point — and it made the core issue obvious. Any read-time projection (whether a dedicated filter or the stock option_*/pre_option_* hooks) silently overrides AP's admin UI: user toggles a checkbox, projection returns something else, admin becomes a lie. Two sources of truth, worst version.

New direction: AP's option is canonical. One-way projection into AT only. AP PR closed.

Divergence from the Object_Type pattern is deliberate — noted in the class docblock. Object-type semantics are FOSSE-specific (force short-form everywhere vs defer); post-type selection is not — it's exactly what AP already stores. Same-shape data, reuse the store.

Changes

• src/class-post-types.php — new projector class. Reads activitypub_support_post_types, returns it from atmosphere_syncable_post_types. Falls back to ['post'] on missing/corrupted option.
• fosse.php — new init action that calls Post_Types::register() behind the same class_exists guard used for Object_Type.
• tests/php/Post_TypesTest.php — six tests: default on unset option, value returned when set, empty array preserved (don't resurrect federation the user disabled), upstream default discarded, non-array option falls back to default, register() idempotent.
• sdd/post-type-sync/notes.md — SDD fix record (problem, approach, tasks, decision trail).
• sdd/onboarding-setup-ux/*.md — spec / plan / requirements / planned-decisions amended. fosse_ap_* option plan replaced with direct activitypub_* writes.

Test plan

• composer run-script lint-php clean
• composer run-script test-php — 19/19 pass (6 new)
• pnpm run format:check clean
• Manual: unset activitypub_support_post_types, confirm Atmosphere sees ['post']
• Manual: set activitypub_support_post_types to ['post', 'page'], confirm Atmosphere's filter returns the same
• Manual: uncheck all post types in AP admin, confirm Atmosphere syncs nothing (empty array preserved, no silent fallback)

[Copilot]

Pull request overview

Adds a new FOSSE “Post_Types” projector to make ActivityPub’s post-type allowlist the single source of truth and project it into Atmosphere, and updates the onboarding SDD to reflect the pivot away from fosse_ap_* option projection toward direct activitypub_* option writes.

Changes:

• Introduce Automattic\Fosse\Post_Types to feed activitypub_support_post_types into atmosphere_syncable_post_types (with sane defaults / corruption handling).
• Register the new projector from fosse.php alongside the existing Object_Type registration guard.
• Update onboarding SDD docs (spec/plan/requirements/planned-decisions) and add a decision-trail note doc for the ownership pivot.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/class-post-types.php	Adds the post-type projector that sources AP’s option and returns it for Atmosphere’s filter.
fosse.php	Registers the new projector on init behind a class_exists guard.
tests/php/Post_TypesTest.php	Adds unit tests for defaulting, passthrough, empty array behavior, and registration behavior.
sdd/post-type-sync/notes.md	Records the decision trail and implementation approach for the ownership pivot + projection.
sdd/onboarding-setup-ux/spec.md	Updates the spec from pre_option_* projection to direct writes to AP options + one-way AT projection.
sdd/onboarding-setup-ux/requirements.md	Revises the resolved “AP settings ownership” decision to direct activitypub_* writes.
sdd/onboarding-setup-ux/planned-decisions.md	Documents the superseded projection plan and the rationale for the direct-write pivot.
sdd/onboarding-setup-ux/plan.md	Updates implementation tasks to write activitypub_* directly and removes pre_option_* projection steps.

[kraftbj]

@RCowles FYI on the SDD amendment.

[kraftbj]

Converting to draft: Codex's adversarial review surfaced a high-severity rollback hazard in the downstream Atmosphere path (narrowing activitypub_support_post_types would orphan Bluesky records for already-synced posts of the removed types, because Atmosphere's on_status_change and on_before_delete gate cleanup on the same allowlist used for publish eligibility).

The projector in this PR is correct; the bug lives in Atmosphere and is fixed upstream at Automattic/wordpress-atmosphere#38 (split the publish-time and cleanup-time gates, three PHPUnit cases added).

Parking this PR in draft until that lands. Follow-up work (bundled re-sync, FOSSE-side regression test, taking this PR out of draft) tracked at DOTCOM-16909. Decision trail captured in sdd/post-type-sync/notes.md.