Moderation notice writer: fence resurface, fix hide-clear/dedup ordering, publish snapshot #6770

Follow-up flagged during review of #6713 (DM resurfacing). This is pre-existing code not touched by that PR (moderation_notices.rs is not in its diff), tracked separately.

Problem

send_moderation_notice clears the recipient hide before duplicate detection (crates/buzz-relay/src/handlers/moderation_notices.rs:121-142), while notice insertion happens later (moderation_notices.rs:157-175):

• A delivered notice → user re-hides → retry of the same moderation_source clears the newer hide, then returns as a duplicate without accepting new activity.
• A failure after the clear but before insertion has the same destructive shape (visibility mutated with no accepted message).
• The accepted path ends at insert/fan-out and never invokes the owner snapshot publisher or the causally fenced resurface helper, so snapshot consumers can stay hidden after a genuinely new safety notice.

Proposed work

• Move the hide-clear after duplicate detection so a duplicate-after-rehide is a no-op.
• Ensure no pre-insert failure can mutate visibility (make acceptance + fenced resurface atomic).
• Use the same causally fenced resurface helper as ordinary ingest / workflow send.
• Publish a visibility snapshot on accepted notices.

Acceptance / tests

• Duplicate-after-rehide is a no-op (newer hide preserved).
• Pre-insert failure performs no visibility clear (rollback / no-clear).
• Accepted new notice updates canonical visibility and publishes the snapshot.

Ref: Jude review at head d28c8b0f4, item #2. #6713