chore: implement phase 7

[Tofel]

Implements proveCoverage (invariant H3) — the pure coverage function: nine checks over one rule's polls (sentinel, from-bounds, heartbeat continuity, health error/nodata, liveness, pause, absence, KeepLast). Liveness is absolute, never a delta; cross-domain comparisons translate by each poll's own skew.

Review focus: coverage.go — the nine checks and the fail-closed skew translation.

[github-actions]

⚠️ API Diff Results - github.com/smartcontractkit/chainlink-testing-framework/grafana-alertcheck

⚠️ Breaking Changes (1)

package github (1)

• com/smartcontractkit/chainlink-testing-framework/grafana-alertcheck/cmd/grafana-alertcheck — 🗑️ Removed

✅ Compatible Changes (1)

package github (1)

• com/smartcontractkit/chainlink-testing-framework/grafana-alertcheck/cmd — ➕ Added

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[Copilot]

🟡 Changes recommended

The new coverage logic has confirmed fail-open edge cases under corrupted log data (e.g., negative skew bounds and future lastEvaluation) that should be handled explicitly before approval.

Once you've addressed the issues Copilot identified, you can request another Copilot review.

Pull request overview

This PR adds the “pure coverage” evaluation layer (proveCoverage) for Grafana alert rule polling, implementing invariant H3 via nine ordered checks over a single rule’s poll stream. It’s designed to be deterministic and side-effect-free, enabling tight unit testing and keeping I/O (HTTP/log reads) out of the coverage decision path.

Changes:

• Introduces proveCoverage and helper functions to compute window membership, heartbeat gaps, health runs, liveness (absolute staleness), pause/absence detection, and KeepLast notes.
• Defines CoverageResult + UnobservableReason vocabulary to produce machine-readable coverage outcomes with human diagnostics.
• Adds a comprehensive unit test suite covering all nine checks, skew translation behavior, multi-failure ordering, and timing derivation round-trip behavior.

File summaries

File	Description
grafana-alertcheck/internal/gate/coverage.go	Implements the pure per-rule coverage proof function and its helper routines.
grafana-alertcheck/internal/gate/coverage_test.go	Adds extensive tests pinning expected behavior for each coverage check and key edge cases.

Review details

Suppressed comments (1)

grafana-alertcheck/internal/gate/coverage.go:302

• ruleHeartbeatGap widens boundary segments by p.SkewBound(), but SkewBoundMS is read from logs without validation (ReadLog is deliberately “crudest possible”). If a corrupted/hand-edited log contains a negative skew_bound_ms, adding it here would shrink the boundary gap (fail-open) instead of widening it. Consider normalizing the bound to a non-negative value before using it in the boundary-gap math.

	runnerOf := func(p Poll) time.Time { return p.GrafanaNow.Add(-p.Skew()) }

	first := in[0]
	if gap := runnerOf(first).Sub(from) + first.SkewBound(); gap > largestGap {
		largestGap, largestGapAt = gap, from
	}

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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