Audit remaining C# 7.x diagnostic coverage and ownership

This is an audit, not a writing task. After the shipped tuple and expression-form work, the remaining risk is inventory drift: orphaned C# 7.x diagnostics, codes still duplicated in the catch-all article despite already having a real owner, or newly discovered adjacent codes that never made the original planning inventory.

Rebuild a definitive inventory for the original August C# 7.x planning set, mapping each code to a shipped owner, a planned September owner, deferred Theme 06, or a newly identified gap. Cross-check the catch-all article against shipped thematic owners (`tuple-errors.md`, `expression-form-restrictions.md`, `expression-tree-restrictions.md`) to confirm nothing lingers there unnecessarily. Flag any newly discovered adjacent diagnostics found during verification.

Deliver a definitive ownership inventory plus, for every newly found item, a concrete recommendation for Bill: fold into Theme 06, attach to an existing shipped article as cleanup, or schedule as a new follow-on theme. Distinguish true gaps from intentionally deferred Theme 06 items.

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[Associated WorkItem - 632365](https://dev.azure.com/msft-skilling/Content/_workitems/edit/632365)