Decision 120: what a net11.0 target must keep, and what waits for 3.0.0 #169

Refines decision 44; 3.0.0 stays timed to .NET 12 LTS (the net10.0 drop, and a change of an existing contract); a union new on net11.0 is an addition that may join a 2.x minor (measured 2026-10-29: a LangVersion 14 consumer who matches on it gets CS9327, and on Metalama 2026.1 CS8652).

A dependency floor may rise in a 2.x minor, on every target or on net11.0 alone, when it is documented: the Semantic Versioning FAQ calls a dependency update with an unchanged public API compatible. Measured with SDK 11.0.100-rc.1: a net11.0 group whose floor is higher than the net10.0 group's fails an application on net11.0 that holds the lower version with NU1605, so the changelog names the dependency, the floors, who it reaches and both ways out, and the smoke test shows it. Metalama is one version for every target; 2.3 and 2.4 stay on 2026.1 and 2.5.0 moves to 2027.0 (#173).

Deliverable: the decision record, its index row and the roadmap's net11.0, Metalama and 3.0.0 items. No code.