Marketplace: item diff, manager, refresh and update detection

Adds diff::item_diff (similar), MarketplaceManager with snapshots,
persisted sync reports, the gh-login slot and a repo lock held across
fetches (pre-flight F11a), refresh_marketplace, load_cached_snapshot,
compute_updates, pins_by_marketplace, head_for, and the GitFixture test
helper on top of git::test_support (F3). AppState gains marketplace.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

app/src-tauri/Cargo.lock

@@ -5576,6 +5576,12 @@ version = "0.1.5"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e3a9fe34e3e7a50316060351f37187a3f546bce95496156754b601a5fa71b76e"
 
+[[package]]
+name = "similar"
+version = "2.7.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bbbb5d9659141646ae647b42fe094daf6c6192d1620870b449d9557f748b2daa"
+
 [[package]]
 name = "siphasher"
 version = "0.3.11"
@@ -6569,6 +6575,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sha2",
+ "similar",
  "tar",
  "tauri",
  "tauri-build",