fix(kernel-utils): handle -0 in encodeMetadataEntry

JSON.stringify(-0) produces "0", so -0 and +0 were serialised to the
same metadataKey and incorrectly collapsed into one germ by
collapseEquivalent. Object.is(0, -0) is false, so decomposeMetadata
already treated them as distinct — making the two functions inconsistent.
Add -0 as an explicit special case alongside NaN, +Infinity, -Infinity.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: grypez

packages/kernel-utils/src/sheaf/sheafify.ts

@@ -51,6 +51,9 @@ const encodeMetadataEntry = (key: string, value: unknown): EncodedEntry => {
     if (value === -Infinity) {
       return [key, '-Infinity', null];
     }
+    if (Object.is(value, -0)) {
+      return [key, '-0', null];
+    }
   }
   return [key, typeof value, value];
 };