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/wire_examples.rs

@@ -103,7 +103,7 @@ fn every_example_round_trips() {
                 round_trip::<ErrorBody>(&name, &example);
                 checked += 1;
             } else if dir_name == "push" {
-                // FCM payloads (SPEC §14.6) are built by M4's sender, not by a wire type of this crate.
+                // FCM payloads (SPEC §14.6) are built by the push sender, not by a wire type of this crate.
                 continue;
             } else if check(&name, &example) {
                 checked += 1;