feat(nix): saturate builds, add community cache, pin nixpkgs registry

- Drop auto-optimise-store: slows every build for modest disk savings.
  Run 'nix store optimise' manually if disk pressure ever shows up.
- max-jobs=auto, cores=0: defaults are 1/1, which left most of the box
  idle during large closures (LLVM, protobuf, …).
- Add nix-community.cachix.org as an extra substituter with its public
  key. Big hit-rate boost against nixos-unstable, which is what the new
  user registry points 'nixpkgs' at.
- dot_config/nix/registry.json pins 'nixpkgs' indirect ref to
  github:NixOS/nixpkgs/nixos-unstable, so 'nix shell nixpkgs#foo' is
  fast + reproducible. Project flakes are unaffected — they pin their
  own inputs via flake.lock.

2 files changed, 27 insertions, 2 deletions

diff --git a/dot_config/nix/registry.json b/dot_config/nix/registry.json
new file mode 100644
index 0000000..387ea7a
--- /dev/null
+++ b/dot_config/nix/registry.json
@@ -0,0 +1,17 @@
+{
+  "version": 2,
+  "flakes": [
+    {
+      "from": {
+        "type": "indirect",
+        "id": "nixpkgs"
+      },
+      "to": {
+        "type": "github",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "ref": "nixos-unstable"
+      }
+    }
+  ]
+}
diff --git a/etc/nix/nix.conf b/etc/nix/nix.conf
index 633422b..3325961 100644
--- a/etc/nix/nix.conf
+++ b/etc/nix/nix.conf
@@ -12,8 +12,10 @@ experimental-features = nix-command flakes
 # the daemon prompting. Safe on a single-user laptop.
 trusted-users = root @wheel
 
-# Hard-link identical store paths to save disk after large builds.
-auto-optimise-store = true
+# Use every core for every build. Defaults are 1/1, which leaves most of
+# the machine idle during big closures (LLVM, protobuf, …).
+max-jobs = auto
+cores = 0
 
 # Keep build outputs and derivations alive even when only referenced
 # from GC roots like direnv envs. Without these, `nix store gc` would
@@ -23,3 +25,9 @@ keep-derivations = true
 
 # Lift the default 64 MiB download buffer; reduces stalls on big closures.
 download-buffer-size = 524288000
+
+# Community-built binary cache. Big hit-rate boost for nixos-unstable
+# packages (which is what the registry pin below points at) and for
+# anything pulled from nixpkgs that already has community CI coverage.
+extra-substituters = https://nix-community.cachix.org
+extra-trusted-public-keys = nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=