docs: document sync test refactoring findings and patterns

- Add comprehensive test pattern guidance to tests/sync/mod.rs
- Explain when to use run_sync_test() vs manual setup
- Document helper scope and architectural limitations

Key findings:
- historic_sync.rs: 4 tests refactored, 143 lines removed (50% reduction)
- live_sync/discovery/tag_variations: Manual setup required due to
  architectural incompatibilities (timing, multi-relay, assertions)
- Helper works for batch historic verification, not real-time scenarios

Detailed summary available in work/sync-test-refactor-summary.md

1 files changed, 108 insertions, 6 deletions

diff --git a/tests/sync/mod.rs b/tests/sync/mod.rs
index ffda73e..58b7354 100644
--- a/tests/sync/mod.rs
+++ b/tests/sync/mod.rs
@@ -3,7 +3,7 @@
 //! This module organizes tests for ngit-grasp's proactive sync functionality.
 //! Tests are grouped by sync scenario:
 //!
-//! - Bootstrap sync (relay syncs from pre-configured bootstrap relay)
+//! - Historic sync (relay syncs from pre-configured bootstrap relay)
 //! - Relay discovery (relay discovers other relays from announcement events)
 //! - Live sync (events sync in real-time after connection established)
 //! - Tag variations (testing different Layer 2/3 tag types: a/A/q, e/E/q)
@@ -12,17 +12,119 @@
 //!
 //! # Test Files
 //!
-//! - `bootstrap.rs` - Tests 1, 4: sync from bootstrap relay
-//! - `discovery.rs` - Tests 2, 3: relay discovery from announcements
-//! - `live_sync.rs` - Tests 5, 6, 7: real-time sync after connection
-//! - `tag_variations.rs` - Tests 8, 9: Layer 2/3 tag type coverage
-//! - `catchup.rs` - Test 0: catchup after disconnect (stub, `#[ignore]`)
+//! - `historic_sync.rs` - Bootstrap and replay tests (uses `run_sync_test()` helper)
+//! - `discovery.rs` - Relay discovery from announcements (manual setup required)
+//! - `live_sync.rs` - Real-time sync after connection (manual setup required)
+//! - `tag_variations.rs` - Layer 2/3 tag type coverage (manual setup required)
+//! - `catchup.rs` - Catchup after disconnect (stub, `#[ignore]`)
 //! - `metrics.rs` - Prometheus metrics integration tests
 //!
+//! # Test Patterns
+//!
+//! This module uses two main testing approaches, each suited to different scenarios:
+//!
+//! ## Pattern 1: Helper-Based Tests (Historic Sync)
+//!
+//! **Use `run_sync_test()` for:**
+//! - Verifying historic event sync (events published before relay starts)
+//! - Bootstrap and initialization tests
+//! - Simple count-based event verification
+//! - Single-relay scenarios
+//!
+//! **Example from `historic_sync.rs`:**
+//! ```rust
+//! use common::sync_helpers::{run_sync_test, build_layer2_issue_event};
+//!
+//! #[tokio::test]
+//! async fn test_bootstrap_syncs_existing_layer2_events() {
+//!     let repo_event = /* create repo announcement */;
+//!     let issue1 = build_layer2_issue_event(&repo_event, "Issue 1");
+//!     let issue2 = build_layer2_issue_event(&repo_event, "Issue 2");
+//!
+//!     run_sync_test(
+//!         &[&repo_event],           // Bootstrap events
+//!         &[&issue1, &issue2],      // Events to verify
+//!         2,                         // Expected count
+//!     ).await;
+//! }
+//! ```
+//!
+//! **Helper Architecture:**
+//! - Publishes all events to bootstrap relay before target relay starts
+//! - Automatically starts target relay with bootstrap relay configured
+//! - Verifies event counts after sync completes
+//! - Handles all relay lifecycle management
+//!
+//! ## Pattern 2: Manual Setup Tests (Live, Discovery, Tag Variations)
+//!
+//! **Use manual setup for:**
+//! - Live sync (events published *during* relay operation)
+//! - Multi-relay coordination (discovery chains)
+//! - Detailed event inspection (tag format verification)
+//! - Precise timing control
+//!
+//! **Example from `live_sync.rs`:**
+//! ```rust
+//! #[tokio::test]
+//! async fn test_live_sync_layer2_events() {
+//!     let bootstrap = TestRelay::start().await;
+//!     let target = TestRelay::start_with_bootstrap(bootstrap.url()).await;
+//!
+//!     // Publish AFTER relay is running (live sync)
+//!     let event = build_layer2_issue_event(&repo, "Live Issue");
+//!     client.publish_event(event).await;
+//!
+//!     // Verify with timing control
+//!     wait_for_event_on_relay(&target, &event.id, timeout).await;
+//! }
+//! ```
+//!
+//! **Example from `discovery.rs`:**
+//! ```rust
+//! #[tokio::test]
+//! async fn test_recursive_relay_discovery() {
+//!     // Multi-relay orchestration
+//!     let relay1 = TestRelay::start().await;
+//!     let relay2 = TestRelay::start().await;
+//!     let relay3 = TestRelay::start().await;
+//!
+//!     // relay1 announces relay2, relay2 announces relay3
+//!     // Verify relay1 discovers relay3 through chain
+//! }
+//! ```
+//!
+//! **Example from `tag_variations.rs`:**
+//! ```rust
+//! #[tokio::test]
+//! async fn test_layer2_sync_with_uppercase_a_tag() {
+//!     // Detailed tag format verification
+//!     let event = build_event_with_uppercase_A();
+//!
+//!     // Custom assertions about tag normalization
+//!     assert!(synced_event.tags.contains_uppercase_a());
+//! }
+//! ```
+//!
+//! ## Why Two Patterns?
+//!
+//! The `run_sync_test()` helper embodies a specific pattern:
+//! ```
+//! Setup → Publish Batch → Start Relay → Verify Counts
+//! ```
+//!
+//! This pattern is **incompatible** with tests needing:
+//! - Event publication *during* relay operation (live sync)
+//! - Multiple relay coordination (discovery)
+//! - Detailed event inspection beyond counts (tag variations)
+//! - Precise timing control
+//!
+//! For these scenarios, manual setup provides necessary flexibility.
+//!
 //! # Shared Imports
 //!
 //! All sync tests use helpers from `common::sync_helpers`:
 //! - `TestClient` - Client with retry logic
+//! - `run_sync_test()` - Helper for historic sync tests
 //! - Event builders for Layer 2/3 events
 //! - `wait_for_event_on_relay()` - Non-panicking assertion helper
 //! - `fetch_metrics()` - Prometheus metrics fetching