Make hunk and lazydiff available outside the Mac

[alycda]

Both hunk and lazydiff were installed through macOS-only paths, so
neither has ever existed in the Linux containers — which is where I do
most of the diff reviewing they're for.

Two independent fixes, one commit each.

hunk: Homebrew → nixpkgs

It was a brew in darwin/modules/homebrew.nix, and nix-darwin's Homebrew
module only runs on Darwin. But nixpkgs carries hunk at 0.17.7 — the
same version the formula ships — with aarch64-linux and aarch64-darwin
both in meta.platforms. Moved to lib/core-packages.nix, so the Mac keeps
it and the containers gain it. Net effect is one fewer Homebrew dependency.

lazydiff: install script → real derivation

It lived only in profiles/work.nix as an activation script piping
upstream's installer into sh.

Moving that to common.nix unchanged would have broken the arm64
container: the installer's platform case block knows only
linux-x86_64, macos-arm64 and windows-x86_64, so on aarch64 Linux it
hits error "unsupported OS/Arch: linux/arm64"; exit 1, and the old
|| exit 1 guard promotes that to a hard activation failure. Same shape as
the base-image collision lesson in CLAUDE.md — nix build stays green
because activation only runs on the target machine.

The binary was there all along: the release publishes
lazydiff-linux-aarch64.tar.gz and a musl variant. The install script is
just stale relative to its own assets. So modules/tools/lazydiff.nix
fetches the asset directly:

• stdenvNoCC + fetchurl, per-system sha256s taken from the .sha256
  files published next to each tarball
• Linux uses the static musl builds — no interpreter or RPATH to patch,
  so dontPatchELF/dontStrip leave them as shipped
• sourceRoot = ".", since the tarball is a bare binary with no wrapping dir
• unsupported systems throw a named error at eval rather than failing
  obscurely mid-activation

work.nix drops its sessionPath and activation block; the module is
imported from common.nix so every profile gets it.

Still alpha, so expect to re-pin — the version string and four hashes are
the only things to touch.

Verification

• nix flake check --all-systems passes
• statix and deadnix clean
• both derivations build and run on aarch64-linux
  (hunk --version → 0.17.7; lazydiff runs)
• both resolve in alyssa@dev and alyssa@work-dev

One quirk recorded in a code comment: lazydiff --version self-reports
0.1.0-alpha.16 from the alpha.17 tag — upstream didn't bump the
internal string. The pin is correct; don't chase the printed number.

Not yet activated anywhere — no home-manager switch run, per usual.

🤖 Generated with Claude Code

[github-actions]

⊕ Entity-level changes

darwin/modules/homebrew.nix

Status	Type	Name
Δ	binding	homebrew

home-manager/modules/common.nix

Status	Type	Name
Δ	binding	imports

home-manager/modules/tools/lazydiff.nix

Status	Type	Name
+	orphan	module-level
+	binding	version
+	orphan	module-level
+	binding	assets
+	orphan	module-level
+	binding	asset
+	binding	lazydiff
+	orphan	module-level
+	binding	home.packages
+	orphan	module-level

home-manager/profiles/work.nix

Status	Type	Name
Δ	binding	home

lib/core-packages.nix

Status	Type	Name
Δ	orphan	module-level

Summary: 10 added, 4 modified across 5 files (5 added orphans, 1 modified orphan)

_{functions and classes, not lines · sem}