refactor(shared-manager): tighten canonical identity capture

Build the canonical identity from the stat getCanonicalFile already
takes, instead of a second lstat of the same inode. One syscall less,
and mode, mtime and identity now describe the same moment rather than
two adjacent ones.

Assert in the adoption race tests that the foreign writer never fired.
It writes only when the canonical path is observed empty, so a zero
count states the invariant the fix establishes - the path is never left
without a regular file - instead of only checking the final content.

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tests/unit/shared-manager.test.ts

@@ -686,6 +686,10 @@ describe('SharedManager', () => {
           foreignWriter.restore();
         }
 
+        // The writer only fires when the canonical path is observed empty, so
+        // zero writes is the invariant itself: adoption never left the path
+        // without a regular file.
+        expect(foreignWriter.placeholderWrites()).toBe(0);
         expect(fs.existsSync(canonicalPath)).toBe(true);
         expect(fs.readFileSync(canonicalPath, 'utf8')).not.toBe(placeholder);
         expect([divergedSettings, previousSettings]).toContainEqual(readJson(canonicalPath));