fix: send relay Follow object as the full Public IRI

[icco]

relay.toot.io and relay.intahnet.co.uk weren't dead and weren't refusing us on policy — we were sending a Follow they can't parse.

Cause

Both run YUKIMOCHI Activity-Relay, both auto-accept (toot.io lists 1,324 connected instances), and both Rejected within 3.5 minutes of a fresh, correctly-formatted Follow after #139 deployed. Its executeFollowing validates with a literal string comparison:

case contains(activity.Object, "https://www.w3.org/ns/activitystreams#Public"):
    // contains(string) → entry == key
...
default:
    return errors.New("only https://www.w3.org/ns/activitystreams#Public is allowed to follow")

What we put on the wire:

type  : Follow
actor : https://robot.villas/users/nyt_homepage
object: "as:Public"

"as:Public" != "https://www.w3.org/ns/activitystreams#Public" → falls to default → Reject. Semantically we're right — the CURIE expands to that IRI — but the relay string-matches, so we lose.

Fedify hit this class of bug against Lemmy and shipped the same rewrite in 2.2.0 (#710), but only for to/cc/bto/bcc/audience, not object. We're on 2.3.4, so there's nothing to upgrade to.

Fix

RelayFollow overrides toJsonLd to emit the full IRI. Doing it inside toJsonLd rather than post-serialization keeps the Object Integrity Proof over the same bytes that go on the wire — the same ordering constraint #710 calls out.

Applied in both places a relay Follow is produced: the sent activity, and the Follow object dispatcher (the relay re-fetches that URL to verify the subscription, and applies the same check).

Testing

• Four new cases: full IRI emitted, clone() preserves the subclass (Fedify clones internally), non-Public objects untouched, and one asserting plain Follow still compacts to as:Public so the upstream behaviour this works around is pinned.
• 174 tests pass against real Postgres.

Real verification is the relays themselves: after deploy, reset the two rows to pending and watch for Accept.

[Copilot]

Pull request overview

This PR fixes relay subscription failures against YUKIMOCHI Activity-Relay instances by ensuring outgoing Follow activities serialize object as the full ActivityStreams Public IRI (https://www.w3.org/ns/activitystreams#Public) rather than the compacted JSON-LD CURIE (as:Public), which some relays reject via literal string comparison.

Changes:

• Introduces RelayFollow, a Follow subclass that rewrites object during toJsonLd() serialization to emit the full Public IRI.
• Switches relay-follow creation paths to use RelayFollow (both the sent activity and the Follow object dispatcher used for relay verification re-fetch).
• Adds tests covering serialization behavior, clone() preserving the subclass, and non-Public objects remaining untouched.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/lib/subscriptions.ts	Adds RelayFollow and the AS_PUBLIC constant to enforce full-IRI serialization for relay follows.
src/lib/federation.ts	Uses RelayFollow when generating relay subscription Follow activities and when serving Follow objects for relay verification.
src/lib/tests/subscriptions.test.ts	Adds unit tests validating RelayFollow serialization, clone behavior, and pinning upstream Follow compaction behavior.

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