feat(observability): report the post-copy verify pass as its own engine state

[aparajon]

Why this matters

The post-copy verify pass compares the copied data against the source before cutover, and on a large table it runs for hours. It has no way to cross the engine interface: engine.State has no checksum member, and progressState returns the tracked state without consulting the engine's phase at all, so every post-copy phase flattens to Running.

Everything downstream is already built for it — the checksumming task state, its proto enum, the PR comment, the CLI, the aggregate counter and the previews all handle a verifying table. The state simply never arrives to populate them, so the operator watches a full copy bar with no ETA and nothing saying a verification is under way.

What it does

Adds StateChecksumming and turns progressState into an explicit engine-phase to engine-state mapping. Phases that flatten into the tracked state are listed rather than left to a default, so giving one of them its own vocabulary is a decision made in that mapping. The secondary-index rebuild is the next candidate: same shape of invisible multi-hour phase.

Stacks on top of the display-side work — this is the producing half.

🤖 Generated with Claude Code

[Copilot]

Pull request overview

This PR improves operator observability of long-running Spirit schema changes by surfacing the post-copy verify pass as a distinct engine state (checksumming) and fixing Spirit table progress projection so verify-phase counters and details are rendered correctly after row copy completes.

Changes:

• Add engine.StateChecksumming and map Spirit’s status.Checksum phase to that state via an explicit phase→engine-state mapping.
• Fix Spirit table progress rendering to treat IsComplete as “row copy complete” (not “table complete”), retire copy ETA after copy completion, and project runner-wide checksum counters/details during verify.
• Extend tern state conversion + tests to persist checksumming as a canonical task state and validate state aggregation behavior.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
pkg/tern/state_converters.go	Persists the new engine checksumming state as canonical task state (state.Task.Checksumming).
pkg/tern/state_converters_test.go	Adds coverage asserting checksumming is in-flight and aggregates to an in-flight apply state.
pkg/engine/spirit/spirit.go	Projects Spirit verify phase into engine state and corrects table progress/detail behavior past row copy completion.
pkg/engine/spirit/spirit_test.go	Adds tests covering checksumming state surfacing and verify-pass table progress projection.
pkg/engine/engine.go	Introduces StateChecksumming as a first-class engine state (non-terminal).

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