Add outbound Fediverse activity handlers

[gabek]

The SDKs gain named handlers for raw outbound ActivityPub notifications.

• Add JavaScript onFediverseOutbound, the matching Python decorator, constants and TypeScript declarations.
• Document the separate fediverse.outbound permission, including exposure of ordinary private activity payloads and exclusion of authentication OTP messages.
• Add paired observer examples that select public Create(Note) activities using explicit audience checks.

These are convenience APIs. Existing SDKs can subscribe with JavaScript on: { "fediverse.outbound": handler } or Python @plugin.on("fediverse.outbound") once the host supports the event.

Host support is in Owncast #5207. SDK CI selects an open Owncast PR with the same branch name, otherwise it uses develop. The initial runs started before #5207 existed and therefore used develop. Rerun both full example workflows to rebuild their host toolchains against the matching core branch. This remains a draft while the core contract is under review. Both observer suites passed 3/3 scenarios locally with the rebuilt host engines.

Companion documentation: owncast.github.io #249.