build: adopt static-analysis packs at suggestion (#15, increment 1)

Add Roslynator, SonarAnalyzer.CSharp, Meziantou.Analyzer, and AsyncFixer as central
analyzer references (Directory.Build.targets, CPM-versioned, guarded on
ManagePackageVersionsCentrally so the gitignored .mcp tool isn't affected).
.editorconfig defaults dotnet_analyzer_diagnostic.severity to `suggestion` so the packs
surface findings without failing the TreatWarningsAsErrors (TWAE) build; rules are
promoted to warning/error incrementally (promotion = enforcement via the existing TWAE
build, so no separate lint step is needed).

StyleCop.Analyzers is intentionally excluded: its latest stable (1.1.118) crashes
(AD0001) on C# records and overlaps the existing .editorconfig/Roslynator.

Blazor .razor: editorconfig severity overrides don't reach analyzer diagnostics in Razor
@code (source-generator limitation; dotnet format can't fix them either), so the
currently-firing SonarAnalyzer rules are temporarily NoWarn-ed in ErsatzTV.csproj and
burned down rule-by-rule in #25. The same rules run at suggestion on .cs.

Formatting normalization (mixed UTF-8 BOM + whitespace) is deferred to its own PR.

Full-solution Release build green (0 errors, 0 warnings).

Refs #15

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.editorconfig

@@ -104,3 +104,19 @@ ij_json_wrap_long_lines = false
 [*.cs]
 # disable CA1848: Use the LoggerMessage delegates`
 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.
+dotnet_analyzer_diagnostic.severity = suggestion
+
+# 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]
+dotnet_analyzer_diagnostic.severity = suggestion
+
+[*.cshtml]
+dotnet_analyzer_diagnostic.severity = suggestion