Show failed derivations in compact error output

[czawistowski-nvidia]

When running a large build such as nixpkgs, nom can spend its limited error area on repeated Nix diagnostics and log tails. Under vertical pressure, it can be hard to find the drv path for true build failures (as opposed to dependency-failed).

This change:

• lists failed .drv paths with their failure types;
• places the failed-build list above the totals so it remains visible when the dependency graph is cropped.

┏━ Dependency Graph:
 ⋮
┣━ 2 Failed Builds:
┃ ⚠ /nix/store/00000000000000000000000000000000-example-one.drv (exit code 1)
┃ ⚠ /nix/store/11111111111111111111111111111111-example-two.drv (hash mismatch)
┃ 15 dependent failures omitted
┣━━━ Builds
┗━ ∑ ...

This retains enough information to inspect a failure with nix log /nix/store/...drv without filling the display with repeated build logs.

[czawistowski-nvidia]

I'm not normally a Haskell-writer and GPT 5.6 Sol did the driving here. I tried to ensure the diff stayed small. Please let me know if you have any style concerns.