chore(spectre): migrate Stellar scan connector to streaming API

[xJeffx23]

Context

PR #34 introduced scanAnnouncementsStream and fetchAnnouncementsStream for Stellar.
Spectre agents currently use the deprecated scanAnnouncements + fetchAnnouncements path which loads all announcements into memory.

Task

Replace Spectre's Stellar scan connector with:

for await (const match of scanAnnouncementsStream(
  fetchAnnouncementsStream('stellar'),
  viewingKey,
  spendingPubKey,
  spendingScalar,
  { window: 64 },
)) {
  handleMatch(match);
}