feat: announcement purgatory

Extends purgatory to hold repository announcements until git data arrives,
preventing empty repositories from being served to clients.

When an announcement is received, a bare repo is created immediately and the
announcement is held in purgatory. It is only promoted and served once a git
push confirms real content exists. If no push arrives before expiry, the bare
repo is deleted and the announcement is silently discarded.

Key behaviours:
- Soft expiry: announcements are hidden from clients but kept alive while git
  pushes are in progress, reviving on successful push
- Expiry is extended when a matching state event or git push is observed
- NIP-09 deletion events remove announcements from purgatory
- Purgatory state (announcements, state events, PR events, expired set) is
  persisted to disk on graceful shutdown and restored on startup, with elapsed
  downtime subtracted from expiry deadlines
- Purgatory announcements drive StateOnly sync in the sync system so state
  events are fetched from listed relays before promotion
- SyncLevel added to RepoSyncIndex to distinguish purgatory repos (StateOnly)
  from promoted repos (Full L2+L3 sync)

1 files changed, 4 insertions, 6 deletions

diff --git a/tests/sync/mod.rs b/tests/sync/mod.rs
index 400341f..70c6981 100644
--- a/tests/sync/mod.rs
+++ b/tests/sync/mod.rs
@@ -82,14 +82,12 @@
 //! **Example from `discovery.rs`:**
 //! ```rust
 //! #[tokio::test]
-//! async fn test_recursive_relay_discovery() {
+//! async fn test_discovers_layer3_via_layer2() {
 //!     // Multi-relay orchestration
-//!     let relay1 = TestRelay::start().await;
-//!     let relay2 = TestRelay::start().await;
-//!     let relay3 = TestRelay::start().await;
+//!     let relay_a = TestRelay::start().await;
+//!     let relay_b = TestRelay::start_with_sync(None).await;
 //!
-//!     // relay1 announces relay2, relay2 announces relay3
-//!     // Verify relay1 discovers relay3 through chain
+//!     // relay_b receives announcement listing relay_a, discovers and syncs from it
 //! }
 //! ```
 //!