feat(sync): implement pagination for historic_sync REQ+EOSE flow

Add automatic pagination support for non-Negentropy historic sync to handle
large result sets efficiently. When a subscription receives >= 75 events,
the system automatically fetches the next page using the 'until' parameter.

Changes:
- Add PaginationState struct to track event counts and min timestamps
- Add pagination_state HashMap to PendingBatch for per-subscription tracking
- Add PAGINATION_THRESHOLD constant (75 events)
- Pass pending_sync_index to event processor for state updates
- Track events and timestamps as they arrive
- Check threshold on EOSE and launch follow-up subscriptions
- Initialize pagination state when creating historic sync subscriptions
- Update test fixtures in algorithms.rs

The pagination continues recursively until a page returns fewer than 75 events,
ensuring complete historic data retrieval without overwhelming relay limits.

1 files changed, 2 insertions, 0 deletions

diff --git a/src/sync/algorithms.rs b/src/sync/algorithms.rs
index a6e0787..f4b1f5c 100644
--- a/src/sync/algorithms.rs
+++ b/src/sync/algorithms.rs
@@ -401,6 +401,7 @@ mod tests {
                 },
                 outstanding_subs: HashSet::new(),
                 sync_method: SyncMethod::ReqEose,
+                pagination_state: HashMap::new(),
             }],
         );
 
@@ -512,6 +513,7 @@ mod tests {
                 },
                 outstanding_subs: HashSet::new(),
                 sync_method: SyncMethod::ReqEose,
+                pagination_state: HashMap::new(),
             }],
         );