chore(release): bump version to 1.8.4-alpha

Left uncommitted by an aborted create-release run on 2026-08-19: the
version bump landed in the tree but the release never reached its tag or
manifest. Committing it so the tree is clean before the release is re-cut.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

core/Cargo.lock

@@ -104,7 +104,7 @@ dependencies = [
 
 [[package]]
 name = "archipelago"
-version = "1.8.3-alpha"
+version = "1.8.4-alpha"
 dependencies = [
  "anyhow",
  "archipelago-container",

core/archipelago/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "archipelago"
-version = "1.8.3-alpha"
+version = "1.8.4-alpha"
 edition = "2021"
 license.workspace = true
 description = "Archipelago Bitcoin Node OS - Native backend"