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/InstanceIsolationTests.cs

@@ -202,7 +202,7 @@ public sealed class InstanceIsolationTests
         // DataRoot / WebViewDataFolder are present in both configurations. Note a
         // const like YTLIVE_INSTANCE is INLINED at compile time and therefore never
         // appears in either binary — grepping for the variable name proves nothing.
-        var source = File.ReadAllText(RepoFile("Helpers", "InstanceProfile.cs"));
+        var source = SourceAssert.Text("Helpers", "InstanceProfile.cs");
 
         // The whole promise is that a Release build compiles to the #else arm, so assert
         // that arm IS production: the one real user profile, no alternate folder, and no
@@ -225,16 +225,4 @@ public sealed class InstanceIsolationTests
         Assert.Contains("instances", debugArm);
     }
 
-    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}");
-    }
 }