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/push_rules.rs

@@ -5,7 +5,7 @@ use axum::http::Method;
 use serde_json::{Value, json};
 
 use crate::common::*;
-use crate::m4::*;
+use crate::sync_fixtures::*;
 
 /// A change applied to a pushed entity.
 type Mutator = Box<dyn Fn(&mut Value)>;