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/main.rs

@@ -1,5 +1,8 @@
-//! Integration tests of the M2a server skeleton: migration identity and pragmas, routes, authentication and the
-//! protocol rule, error shapes, and the wire types against `contracts/examples/`.
+//! Base server tests: migration identity and pragmas, routes, authentication and the protocol rule, error shapes,
+//! and the wire types against `contracts/examples/`.
+
+#[path = "../common/mod.rs"]
+pub mod common;
 
 mod errors;
 mod migration;