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/main.rs

@@ -1,13 +1,14 @@
-//! M4 sync integration tests: every rule of SPEC §14.4.2 (pull) and §14.4.4 (push steps 1 to 12), each rejection
+//! Sync integration tests: every rule of SPEC §14.4.2 (pull) and §14.4.4 (push steps 1 to 12), each rejection
 //! reason of §14.4.6, the §9.10 conflict cases, batch limits, `cursor_ahead`, the epoch, the 14-day offline scenario
 //! (AC-15.6), the audit writer (§14.4.7), `rotate-epoch` (§10.4), the end-to-end path device A push → device B pull,
-//! and the orchestrator-assigned `serve` fixes (ConnectInfo for the rate limiter, `/i/` HTML 404).
+//! and `serve` itself (ConnectInfo for the rate limiter, `/i/` HTML 404, the shutdown drain).
 //!
-//! Reuses M3's harness (`tests/core_api/common.rs`) with the real `SqlAuditWriter` swapped in (`m4.rs`).
+//! The harness is `tests/common/`, with the real `SqlAuditWriter` swapped in by `sync_fixtures`.
 
-#[path = "../core_api/common.rs"]
+#[path = "../common/mod.rs"]
 pub mod common;
-pub mod m4;
+#[path = "../common/sync_fixtures.rs"]
+pub mod sync_fixtures;
 
 mod audit;
 mod conflicts;
@@ -16,9 +17,3 @@ mod offline;
 mod pull;
 mod push_rules;
 mod serve;
-
-impl Default for common::TempDir {
-    fn default() -> Self {
-        Self::new()
-    }
-}