js-rigor: challenge proofs treat zero timestamps as valid; flake root-caused

The transient full-suite flake (fail 1 in ~5-10% of runs, only visible on
some invocations) was finally captured: challenge-proof's most-recent-proof
property failed once in 800 cases. Root cause: _addChallengeProof built
issuedAt with proof.issuedAt || proof.issued_at || proof.timestamp ||
Date.now() — a generated issuedAt of 0 (epoch) was replaced with the wall
clock, so an epoch-issued proof became the most recent one and the
lookup returned a timestamp (1.7e12) that could never match the
expectation. The same ||-chain corrupted expiresAt: 0 into null, making
epoch-expired proofs never expire. All three chains (issuedAt, expiresAt,
proofId) now use nullish coalescing so zero is a valid timestamp.

Fixed pin added; the challenge-proof file went from ~10% flake to 20/20
clean, and the full suite is stable at 801 tests / 739 pass / 0 fail.

src/core/PartialGraphContext.js

@@ -120,15 +120,18 @@ export class PartialGraphContext {
   _addChallengeProof(proof) {
     const subjectId = this._resolveNodeId(proof.subject);
     const now = Date.now();
-    const issuedAt = proof.issuedAt || proof.issued_at || proof.timestamp || now;
-    const expiresAt = proof.expiresAt || proof.expires_at || null;
+    // 0 is a valid timestamp (epoch-issued / already-expired); `||` would
+    // replace it with the wall clock or null, making an epoch-issued proof
+    // the most recent one and an epoch-expired proof never expire.
+    const issuedAt = proof.issuedAt ?? proof.issued_at ?? proof.timestamp ?? now;
+    const expiresAt = proof.expiresAt ?? proof.expires_at ?? null;
     const record = {
       name: proof.name,
       subject: subjectId,
       issuedAt,
       expiresAt,
       source: proof.source || 'partial',
-      proofId: proof.proofId || proof.proof_id || null,
+      proofId: proof.proofId ?? proof.proof_id ?? null,
       metadata: proof.metadata || null
     };
     const key = this._challengeKey(record.name, subjectId);