fix: resolve last remaining analyzer findings (CA1001, CA1031, CA1515, CA1711, CA1822, CA1861)

[DeepDiver1975]

Summary

Closes the analyzer-hardening tail that remains after the merged analyzer series. With AnalysisMode=All, main currently produces exactly 30 warnings (16 unique sites) across six rules — all in the audio-cue service, its test doubles, and PDF table headers. This PR zeroes them so the Directory.Build.props tightening to AnalysisMode=All + TreatWarningsAsErrors can land without breaking CI.

Rule	Count	Fix
CA1001	2	SerialAudioQueue: suppression w/ justification (app-lifetime singleton, BlockingCollection intentionally never disposed)
CA1031	4	SerialAudioQueue.Run + SystemAlarmService.PlayBlocking: suppression w/ justification (pattern already used in this file)
CA1515	2	SerialAudioQueue → internal sealed (already only referenced inside LageBuch.App, tests access via InternalsVisibleTo)
CA1711	2	resolved as a side effect of internalization (reserved-suffix rule is visibility-gated)
CA1822	8	4 test-double IAlarmService.Start()/Stop() impls: suppression w/ justification (interface members cannot be static)
CA1861	12	6 PDF section headers: hoist new[] { … } to private static readonly string[] HeaderTitles

Verification

• vs origin/main (37dae99): build with EnableNETAnalyzers + AnalysisLevel=latest + AnalysisMode=All → only those six rules drop to 0; 0 errors; no other-rule deltas.
• Full suite: 803 passed / 1 skipped.
• Single signed commit, rebased onto current main.