Update horizon-core input, add brick, add missing package overrides (9.12.x)

Refresh horizon-core from 425d9724 (2025-05-10) to 721d0c2f (2026-04-03) — picks up the horizon-package-set-lts-ghc-9.12.x merge and ~11 months of upstream activity.

Adds brick back to the package set and to requiredPackages — it should be required on every lts branch.

The new horizon-core no longer exposes some packages that were previously available via legacyPackages, so they are declared here in horizon.dhall and regenerated via horizon-gen-nix: brick, constraints-extras, dependent-sum, dependent-sum-template, email-validate, formatting, groups, indexed-profunctors, keys, monad-logger, monad-loops, neat-interpolation, optics-core, pointed, quickcheck-text, rebase, rerebase, stm-chans, string-interpolate, text-conversions, vector-instances, vty-crossplatform, vty-unix.

This supersedes !204 (closed) which took a shortcut by hand-writing pkgs/*.nix including stub files with dummy hashes — the stubs have been replaced with horizon-gen-nix output generated from horizon.dhall declarations.