Add SyncContext trait and MockSyncContext for purgatory sync

Implement the abstraction layer for purgatory sync operations:

- SyncContext trait: defines interface for repository data fetching,
  OID existence checks, git fetch operations, and event processing
- ProcessResult: captures outcomes when releasing events from purgatory
- MockSyncContext: test mock with builder pattern for configuring:
  - Clone URLs and which OIDs each URL provides
  - Needed OIDs (simulates purgatory state)
  - URL failure simulation
  - Fetch logging for assertions

The trait uses async_trait for async method support and requires
Send + Sync for use in concurrent sync operations.

This abstraction enables unit testing of sync logic without I/O,
while the real implementation (to be added later) will connect
to actual database, git, and relay systems.

1 files changed, 12 insertions, 0 deletions

diff --git a/Cargo.lock b/Cargo.lock
index 03afcbb..35b96c0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -121,6 +121,17 @@ dependencies = [
 ]
 
 [[package]]
+name = "async-trait"
+version = "0.1.89"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "9035ad2d096bed7955a320ee7e2230574d28fd3c3a0f186cbea1ff3c7eed5dbb"
+dependencies = [
+ "proc-macro2",
+ "quote",
+ "syn",
+]
+
+[[package]]
 name = "async-utility"
 version = "0.3.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -1425,6 +1436,7 @@ name = "ngit-grasp"
 version = "0.1.0"
 dependencies = [
  "anyhow",
+ "async-trait",
  "base64 0.22.1",
  "clap",
  "dashmap",