Clarify score_approx field conditionality in format.md

[wollax]

Problem

The score_approx field is marked required: yes in the diagnostics schema table. The Optionality subsection notes all sub-tables are independently optional, but doesn't clarify that fields within a sub-table entry are required only when that sub-table is present. Could confuse implementors.

Solution

Add clarifying note in schema table or prose: "Fields marked 'yes' are required within each entry when the sub-table is present."

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Created via Kata /kata-add-issue