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/skeleton/routes.rs

@@ -80,7 +80,7 @@ async fn every_route_is_registered_and_implemented() {
         33,
         "SPEC §7 has 33 operations"
     );
-    // M3 implemented E-3 to E-30 and E-33 (tests/core_api), M4 the sync routes (tests/sync_api): nothing is 501.
+    // Every route has its handler (behaviour in tests/core_api and tests/sync_api): nothing is 501.
     for (method, path, _) in routes {
         let reply = app
             .send(authed(method.clone(), &path, &admin_token()))