fix: P0 correctness bugs (halt detection, hangs, panic, docs)

[auricom]

Fixes the remaining P0 issues from the optimization roadmap (#53). The fifth P0 issue, #35 (Celestia auth token), was already fixed on main by #55, so this PR no longer touches the Celestia client.

Changes

#34: time_since_last_block never grows on a halted chain

pkg/exporters/drift/drift.go reset the gauge on every successful poll, so a halt alert could never fire.

• The gauge is now reset only when the reference height goes up (lastHeight). A flat or decreasing height leaves it alone.
• Removed the extra reset that ran once per full node.

#36: process hangs when one exporter fails; --duration exits 1

• pkg/metrics/server.go now uses errgroup.WithContext. When one exporter fails, the HTTP server and the other exporters stop, and the process exits instead of staying up as a zombie.
• cmd/monitor.go now treats context.DeadlineExceeded (from --duration) as a clean exit.

#37: verifier panic at shutdown

pkg/exporters/verifier/verifier.go no longer closes blockQueue. Workers exit on ctx.Done(). This removes a send-on-closed-channel panic that delayed re-queue goroutines could trigger.

#38: README documented flags and modes that don't exist

• Removed --enable-metrics, --debug and "one-shot mode" from the README.
• Fixed the --port help text.
• Rewrote the Debug Logging section to describe what actually exists: --verbose and EVMETRICS_DEBUG.
• Marked block_time_slo_seconds, block_receive_delay_seconds and block_receive_delay_slo_seconds as "not yet emitted, see [P2] Emit declared-but-unused metrics: block_receive_delay and SLO threshold gauges #44".

Tests

New tests for each fix:

• drift_test.go: fails on the old code.
• server_test.go: reproduces the hang without the fix.
• verifier_test.go: workers stop on cancel.

Results, after merging the latest main (#55, #56):

• go test -race ./... passes.
• golangci-lint v2.15.0 reports 0 issues.
• gofmt -l reports nothing.

Note

The drift test uses real 1s polls with about 0.3–0.5s of slack. If it turns flaky in CI, the follow-up is to inject the ticker.

Fixes #34
Fixes #36
Fixes #37
Fixes #38

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