fix(server): keep ready checkpoints when a later placeholder arrives

[Adolanium]

What Changed

The SQL turns projector now refuses a later missing placeholder over a checkpoint that is already ready or error.

In-memory projectEvent already had this guard. SQL always wrote checkpointStatus: event.payload.status. Thread detail and diffs read SQL.

Same-turn placeholder then ready is still the happy path. This only stops a later placeholder from wiping a capture that already landed.

Why

Ingestion and CheckpointReactor dispatch onto the same serial queue. If ready lands first, a later missing was a no-op in memory and a clobber in SQL.

Checklist

• This PR is small and focused
• I explained what changed and why
• I included before/after screenshots for any UI changes (N/A)
• I included a video for animation/interaction changes (N/A)

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Note

Low Risk
Narrow projection guard on duplicate turn-diff events; improves read-model consistency for checkpoints with limited blast radius.

Overview
Aligns the SQL turn projector with in-memory event projection so a later thread.turn-diff-completed with missing cannot overwrite a turn that already has a real checkpoint (ready or error).

When ready is projected before a trailing placeholder (e.g. shared serial queue ordering), projection_turns used to always take the latest status, so thread detail and diffs could show missing even after a successful capture. The handler now no-ops that downgrade; placeholder-then-ready on the same turn is unchanged.

Adds an integration test that projects ready then missing for the same turn and asserts SQL still stores ready and the checkpoint ref.

^{Reviewed by Cursor Bugbot for commit c5e84c9. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix ProjectionPipeline to keep ready checkpoints when a later missing placeholder arrives

• In the thread.turn-diff-completed handler in ProjectionPipeline.ts, adds an early return guard: if an existing turn projection already has a non-null, non-missing checkpointStatus, incoming events with status missing are ignored for that turn.
• Adds a test in ProjectionPipeline.test.ts that projects a ready checkpoint followed by a missing event for the same turn and asserts the ready status and original checkpointRef are retained.

^{Macroscope summarized c5e84c9.}

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[macroscopeapp]

Approvability

Verdict: Approved at c5e84c9

Macroscope's review found this PR approvable — This is a narrow read-model consistency fix: only a later missing checkpoint event is ignored when a real checkpoint already exists, preserving the stored status and git reference. The accompanying integration test covers the affected ordering, with no schema or cross-cutting runtime changes.

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