Epic: 10.0.0 #58

Goal

Fix the defects that answer wrongly without throwing, and the fixes that tighten behaviour. Under the release rule, none of these can ship in a minor version.

In scope and out of scope

In scope: issues labelled silent or breaking, whether they're new or old.

Out of scope: additions and loosening changes. Those ship as 10.x minors after 10.0.0 and are merged forward to the 11 line (#147).

Since 2026-10-18 the 11 line (#147) is open in parallel on a net11 branch. Every fix here lands on main first and is merged forward; nothing is fixed on net11 first.

Done when

• every sub-issue is closed
• the root changelog opens 10.0.0 with a "what can break you" list, as 7.0.0, 8.0.0 and 9.0.0 did, and docs/migration.md has a v9.x section
• v10.0.0 is tagged and the four packages are on NuGet
• release/10.x is cut at the tag and net11 has merged the tag forward (Two supported lines: policy, branch rules, SECURITY rows, and the silent-fix rule as a minor with a changed-answers section #151)