system traces: sameValue JSON.stringify guard can throw on circular/BigInt values #56

TraceProducerBase.emitIfChanged de-dups with a JSON.stringify-based sameValue (packages/capture/src/trace-producer.ts). A circular or BigInt value makes it throw. The provider isolates producer errors, so the trace is dropped rather than crashing, but the value is silently lost.

Fix: a total comparison (structural, or try/catch falling back to "changed"). Test-first with circular and BigInt cases.

Deferred from PR #55 (trace producers over shared trackers).

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