8eade6ee

Update the flake inputs

Bottom of a three-PR stack.

`nix flake update` across every input: nixpkgs (nixos-26.05, 2026-10-07),
nixpkgs-25_11, horizon-advance, pre-commit-hooks, sydtest, validity,
opt-env-conf, safe-coloured-text and fast-myers-diff.

It also drops the `nixos-25.05` backward-compatibility check.  sydtest no
longer builds on the GHC that release ships — `sydtest-mutation-runtime`'s
`Test/Syd/Mutation/AugmentedManifest.hs` uses `foldl'` without importing it,
which only compiles against base >= 4.20 where Prelude re-exports it — and
26.05 is the release this has to work on.  `nixos-25.11` still works, so it
stays.

`nix flake check` passes.

Total

4h50m45s

Created 2 weeks ago Started 2 weeks ago Finished 2 weeks ago

Timeline