docs(shared-manager): note why the CAS mismatch raises EEXIST

The code is this file's marker for "a race was detected" - the same one
the reappeared-path and concurrent-replacement guards raise - not a
report that a no-replace link() or open('wx') hit an existing path. Say
so at the throw, so debugging by err.code does not send anyone looking
for a no-replace failure that never happened.

Built [OnSteroids](https://onsteroids.ai)

src/management/shared-manager/diverged-file-adopter.ts

@@ -264,6 +264,9 @@ function publishCanonicalContent(
     const currentStats = getLstatSync(writePath);
     const current = currentStats?.isFile() ? canonicalIdentityOf(currentStats) : null;
     if (!canonicalIdentityMatches(expected, current)) {
+      // EEXIST is this file's marker for "a race was detected", the same code
+      // the reappeared-path and concurrent-replacement guards raise. It does
+      // not mean a no-replace link() or open('wx') hit an existing path.
       throw Object.assign(new TypeError(`Canonical file changed during adoption: ${writePath}`), {
         code: 'EEXIST',
       });