fix(server): preserve V2 checkpoint baselines across runs

[saphid]

Full-thread diffs lose their baseline after a second V2 run because the reused checkpoint scope is no longer owned by the first run. Resolve ordinal zero from the target checkpoint scope and keep nonzero ranges within that same scope.

Rebuilt head 6cf02a815 on V2 3aa47e294 with test-only prerequisite #10865 (merge first). Two-file topic; regression uses the real ID allocator and projector across two runs.

Validation: all six focused checkpoint query tests pass, plus server typecheck, targeted lint and formatting. New CI pending. Direct Claude Opus 5 high review remains unavailable: latest launch exited 1 on expired OAuth before inference.

Targets V2 #2829. Updated with GPT-6 in Codex/T3.

[macroscopeapp]

Approvability

Verdict: Approved at 6cf02a8

Macroscope's review found this PR approvable — The production change is a small, isolated correction to V2 checkpoint baseline selection for multi-run diffs, backed by focused regression tests. Other modified files are test-only, with no schema, default, deployment, security, billing, or static-analysis changes.

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Dismissing prior approval to re-evaluate 6cf02a8