feat(analyzers): enable incremental latest-All enforcement

Centralize SDK and threading analyzers, baseline the .NET 10 All rule inventory at suggestion severity, and promote S3981 repo-wide. Fix the always-true worker count predicate and cover the idle/active branches.

Fixes #15

Co-Authored-By: Codex <codex@openai.com>

.editorconfig

@@ -106,13 +106,17 @@ ij_json_wrap_long_lines = false
 dotnet_diagnostic.ca1848.severity = none
 
 # --- Static-analysis pack adoption (ersatztv#15) ---
-# Roslynator / SonarAnalyzer / Meziantou / AsyncFixer are referenced centrally
-# (Directory.Build.targets). Default every analyzer diagnostic to `suggestion` so the new
-# packs don't fail the TreatWarningsAsErrors build; high-value rules get promoted to
-# warning/error one at a time (see ersatztv#15 / docs/contributing.md). Explicit per-rule
-# severities (e.g. ca1848 above) still take precedence over this bulk default.
+# Threading analyzers and Roslynator / SonarAnalyzer / Meziantou / AsyncFixer are enabled centrally.
+# Default their diagnostics to `suggestion`; the SDK's exact per-rule suggestion baseline lives in
+# eng/analyzers/sdk-all-suggestion.globalconfig because AnalysisLevel=latest-All otherwise injects
+# exact warning severities that outrank this bulk setting. High-value rules are promoted one at a
+# time. Explicit per-rule severities (e.g. ca1848 above) take precedence over both baselines.
 dotnet_analyzer_diagnostic.severity = suggestion
 
+# A collection count can never be negative. Treat comparisons that therefore collapse to a
+# constant as errors; the first promotion caught a busy/idle branch that was permanently busy.
+dotnet_diagnostic.S3981.severity = warning
+
 # Blazor components: analyzers run on .razor/.cshtml @code too, and TWAE would otherwise
 # turn their default-severity findings into build errors — keep them at suggestion as well.
 [*.razor]