ci: gate Core coverage and exclude test tooling from the report

Coverage settings exclude the fuzz harness, server probe and test assembly;
CI fails if Core drops below 95% lines or 88% branches. The gitignore no
longer swallows tests/coverage.runsettings.

tests/check-coverage.py

@@ -0,0 +1,37 @@
+#!/usr/bin/env python3
+"""Fail if Cobertura line/branch coverage of an assembly drops below a threshold.
+
+usage: check-coverage.py COVERAGE.xml PACKAGE:MIN_LINE[:MIN_BRANCH] [...]
+example: check-coverage.py coverage.cobertura.xml PalladiumWallet.Core:90:80
+Prints a per-package summary either way. Stdlib only (runs on any CI runner)."""
+import sys
+import xml.etree.ElementTree as ET
+
+
+def main() -> int:
+    if len(sys.argv) < 3:
+        print(__doc__)
+        return 2
+    root = ET.parse(sys.argv[1]).getroot()
+    packages = {p.get("name"): p for p in root.iter("package")}
+    failed = False
+    for spec in sys.argv[2:]:
+        name, *limits = spec.split(":")
+        min_line = float(limits[0]) if limits else 0.0
+        min_branch = float(limits[1]) if len(limits) > 1 else 0.0
+        pkg = packages.get(name)
+        if pkg is None:
+            print(f"FAIL {name}: not present in the report")
+            failed = True
+            continue
+        line = float(pkg.get("line-rate")) * 100
+        branch = float(pkg.get("branch-rate")) * 100
+        ok = line >= min_line and branch >= min_branch
+        failed |= not ok
+        print(f"{'ok  ' if ok else 'FAIL'} {name}: lines {line:.1f}% (min {min_line:g}), "
+              f"branches {branch:.1f}% (min {min_branch:g})")
+    return 1 if failed else 0
+
+
+if __name__ == "__main__":
+    sys.exit(main())

tests/coverage.runsettings

@@ -0,0 +1,18 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!-- Used by CI and dotnet test with the settings file tests/coverage.runsettings and the XPlat Code Coverage collector.
+     Excludes the test tooling itself (fuzz harness, server probe, the test assembly) so the
+     figures describe product code only: Core, App and Cli. -->
+<RunSettings>
+  <DataCollectionRunSettings>
+    <DataCollectors>
+      <DataCollector friendlyName="XPlat Code Coverage">
+        <Configuration>
+          <Format>cobertura</Format>
+          <Exclude>[PalladiumWallet.Fuzz]*,[PalladiumWallet.ServerProbe]*,[PalladiumWallet.Tests]*,[Avalonia*]*</Exclude>
+          <ExcludeByAttribute>GeneratedCodeAttribute,CompilerGeneratedAttribute,ExcludeFromCodeCoverageAttribute</ExcludeByAttribute>
+          <SkipAutoProps>true</SkipAutoProps>
+        </Configuration>
+      </DataCollector>
+    </DataCollectors>
+  </DataCollectionRunSettings>
+</RunSettings>