[reka18]

What this change does

A checkpoint now lands on any step that crosses a WithCheckpointEvery boundary. Stacked on #236. This is the top of the #214 chain.

Such an instance now compacts, so on a Store without HistoryStore, StateAt can't reach the steps before its latest checkpoint. It answers with the checkpoint's state, or ErrStepOutOfRange when the checkpoint left the tail empty, as it already did for a run of Fires. The cadence is unchanged for plain steps. A Tick record can now checkpoint on a step other than the usual one.

Why

A Tick record spans several step ordinals, so it could miss the interval forever and the tail grew without limit. A review of #232 found this. It isn't listed in #214.

Still open in #214 after this chain: telemetry on replay (question 7), which #214 defers, and the machine version opt-in (question 6). Questions 3 and 4 need a ruling.

Relates to: #214

Evidence

On GOTOOLCHAIN=go1.27.3, in durable:

Command	Result
go test ./...	pass. TestTimerCheckpoint_BarriersCannotStepOverTheInterval is new, and it fails on the previous commit with a 52-record tail
go test -race ./...	pass, 0 race warnings
Pinned golangci-lint	0 issues
go test ./... in state, cluster, e2e, transport, source, examples/dispatch, examples/fooddelivery	pass

Checklist

• Commits are signed off (git commit -s) per the DCO
• Conventional commit messages (type: subject)
• mage check passes locally (lint + race tests + govulncheck): not run as a whole
• Tests added/updated for the change
• Public API changes documented (godoc) and, if applicable, noted for the changelog

[reka18]

Review results. One reviewer per dimension, each reading the others' findings. All seven dimensions have zero blockers on the latest round.

Fixed along the way: a test the new cadence had stopped exercising now pins a non-empty tail, and the StateAt behavior on compacted steps is disclosed.

Known limit: StateAt below the latest checkpoint returns the checkpoint's state without an error on a Store with no history. That predates this PR and needs its own issue.