tests: one source-assertion helper, so the prose trap can't recur

The trap I hit twice today — a file-wide DoesNotContain matching the doc
comment that explains the ban — was possible because three tests each
hand-rolled their own File.ReadAllText + path-walker, two of them mine. Fixed
at the root rather than per-test: SourceAssert now owns path walking and
literal extraction, with Literal(memberName, params parts) and Scopes() that
assert on the parsed VALUE. Tests that genuinely assert structure (the #if
DEBUG arm in InstanceIsolationTests, the absence of a whole API in the
locator) still read the file themselves, because those claims cannot be
reduced to a value — the helper is for literal facts only.

Shipping the fix introduced two defects, both caught by the build: a method
named Path shadowed System.IO.Path and broke the error message that names the
missing file, and Literal(fileName, memberName) invited a call that silently
resolved to a nonexistent path. Argument order is now required-context-first
so a wrong path is visible at the call site.

Suite 380/380, 0 warnings. LayerReorderPersistenceTests.RealMouseDrag flaked
once mid-session with no ytLive instance running and passed in isolation and
on re-run — the environment flake already recorded in HANDOFF.md, not a
regression from this change.

ytLive.Tests/YouTubeAuthServiceTests.cs

@@ -140,25 +140,9 @@ public class YouTubeAuthServiceTests
     [Fact]
     public void ExactlyOneScopeIsRequested_AndItIsTheBroadOne()
     {
-        var source = File.ReadAllText(RepoFile("Services", "YouTubeAuthService.cs"));
-
-        var match = Regex.Match(source, @"private const string Scope = ""([^""]*)"";");
-        Assert.True(match.Success, "could not find the Scope const in YouTubeAuthService.cs");
-
-        var scopes = match.Groups[1].Value.Split(' ', StringSplitOptions.RemoveEmptyEntries);
-        Assert.Equal(new[] { "https://www.googleapis.com/auth/youtube" }, scopes);
+        Assert.Equal(
+            new[] { "https://www.googleapis.com/auth/youtube" },
+            SourceAssert.Scopes("Scope", "Services", "YouTubeAuthService.cs"));
     }
 
-    private static string RepoFile(params string[] parts)
-    {
-        var dir = new DirectoryInfo(AppContext.BaseDirectory);
-        while (dir != null)
-        {
-            var candidate = Path.Combine(new[] { dir.FullName }.Concat(parts).ToArray());
-            if (File.Exists(candidate)) return candidate;
-            dir = dir.Parent;
-        }
-        throw new FileNotFoundException(
-            $"could not locate {Path.Combine(parts)} above {AppContext.BaseDirectory}");
-    }
 }