Server overhaul: one test harness in tests/common for every test binary

core_api/common.rs becomes tests/common/mod.rs and sync_api/m4.rs tests/common/sync_fixtures.rs; each binary
includes them through #[path] (the binaries of SPEC §14.7 stay as they are). TempDir's Default moves into the
harness instead of three copies; the skeleton helpers take ids, tokens and TempDir from it. Milestone wording in
test docs and the server README replaced by what the code does; README seams and test layout updated.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

server/crates/kvitt-server/tests/sync_api/audit.rs

@@ -1,10 +1,10 @@
-//! The real audit writer (SPEC §14.4.7, FR-12) through M3's recurring generation and through sync: source, actor,
+//! The real audit writer (SPEC §14.4.7, FR-12) through the recurring generation and through sync: source, actor,
 //! times and append-only entries.
 
 use serde_json::json;
 
 use crate::common::*;
-use crate::m4::*;
+use crate::sync_fixtures::*;
 
 #[tokio::test]
 async fn generation_writes_recurring_audit_with_server_now_and_no_device() {