Cover the beat lifecycle (New/Run/Stop) to satisfy the coverage gate

[cjimti]

Closes #14.

Raises beater coverage from ~27% to 89% (repo 86.8%), clearing the >= 80% Codecov gate, with a real lifecycle test rather than coverage padding. make verify passes; go test -race -count=10 is clean.

Test

TestRunLifecycle drives the full beat through a fake libbeat pipeline:
New → Run on an ephemeral port → POST /in (asserts the pipeline receives the published events) → GET /metrics → invoke the captured ACK handler → Stop (asserts the client closes and Run returns nil).

Per-function after this change: Stop 100%, Run ~93%, New 75%, buildEvents/inHandler 100%. The remaining gaps are unreachable error paths (a logger-build failure, log.Fatalf on a bind error).

Production change (behavior-preserving DI seam)

Rtbeat gains optional registerer/gatherer fields, both nil in production, resolving to the global default registerer/gatherer. Metrics are built via promauto.With(registerer); /metrics is served via promhttp.InstrumentMetricHandler(registerer, HandlerFor(gatherer, HandlerOpts{})).

Verified against client_golang v1.23.2 source that promhttp.Handler() is exactly InstrumentMetricHandler(DefaultRegisterer, HandlerFor(DefaultGatherer, HandlerOpts{})) — so with the fields nil the prod /metrics output (go_*, process_*, promhttp_metric_handler_*, and the four rtbeat_* metrics) is unchanged. The seam exists so tests can inject a private registry; otherwise a second Run in one process would panic on the global registry's duplicate-registration guard (a fragility this PR also removes).

Review

Adversarial review confirmed prod equivalence (against the actual module source), a race-free/non-flaky test, and meaningful assertions. A naive HandlerFor-only handler would have dropped two promhttp_metric_handler_* metrics from prod — caught and corrected to use InstrumentMetricHandler.

[codecov]

Codecov Report

❌ Patch coverage is 71.42857% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.99%. Comparing base (ccb809e) to head (75e7de2).

Files with missing lines	Patch %	Lines
beater/rtbeat.go	71.42%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #15       +/-   ##
===========================================
+ Coverage   25.43%   86.99%   +61.55%     
===========================================
  Files           2        2               
  Lines         114      123        +9     
===========================================
+ Hits           29      107       +78     
+ Misses         85       10       -75     
- Partials        0        6        +6     

Files with missing lines	Coverage Δ
beater/rtbeat.go	89.16% <71.42%> (+63.04%)	⬆️

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