Support shared assertions across case results (each_result / common_traces)

[SchuylerGoodman]

Problem

When multiple test cases in a spec all call the same operation, the harness spec test cases must repeat the same traces: block for every case result. This is verbose and error-prone.

Example

13 scalar_operators cases all produce the same 4 trace events. The harness spec repeats those 4 lines 13 times.

Proposed solution

A common_traces or each_result field at the results level that defines assertions valid for all results:

expected:
  outcome:
    Complete:
      each_result:
        traces:
          - $run: get_measurement
          - temperature: { $exists: true }
      results:
        - name: regex_match
          status: pass
          expected:
            - label: { $matches: "^sensor" }

This eliminates repetition while still proving operations ran.