Interop suite, release-link CI and 0.7.0 (DEV-1150 part 3d)

[adamski]

Interop and release for the v2 redesign (DEV-1150 part 3d). Releases replicant-client 0.7.0, which requires replicant-server 0.5.0 (tagged on facd572).

What changes

• Interop harness restored for protocol v2. test/run_phoenix_interop_local.sh runs against replicant-server at facd572, using its own throwaway database (INTEROP_DB_NAME, default replicant_interop_v2).
• v2 interop suite (tests/interop.rs), run against the real server:
  • first sync events
  • create, update and delete across devices
  • offline edits
  • field merges and field conflicts
  • canonical numbers
  • curated publications
  • auth halts
  • identity adoption
  • silent-drop detection (about 60 s) and recovery
• Two processes on one data dir (tests/two_process.rs):
  • an offline edit is seen in under 1 s;
  • concurrent uploads converge;
  • whole-content writes end with one server state.
• CI:
  • an interop job, which runs the same 17 top-level tests as the harness (11 interop, 3 two-process, 3 smoke);
  • a release-link job on macOS, Linux and Windows, which checks the version marker survives release links;
  • a macOS thread-count check.
• .gitlab-ci.yml is removed (it built the removed Rust server).
• An ignored probe runs migration 015 over a copy of a v1 database.
• Version bumped to 0.7.0.

No engine behaviour changes.

What 3d found

• Real-library migration: a real v1 library migrates and syncs cleanly. All 190 documents match the server, there are no kept copies, and a restart changes nothing.
• Defects: no engine or server defects.
• Follow-ups filed:
  • v1 offline edits migrate as creates and come back as conflict kept copies, even when the server is unchanged #54: v1 offline edits come back as conflict kept copies even when the server is unchanged. No data is lost; hosts must show list_recovered.
  • Timing test repeated_attach_and_close_cycles can exceed its bound under extreme load #55: a timing test can exceed its bound under extreme load.

To check on this PR's CI

release-link on Windows is the one platform not verified locally. It checks that the version marker survives /OPT:REF.

After merge: tag v0.7.0 on the merge commit.