refactor(flake): expose lib.mkBlitzApi for consumer-pkgs builds

Extract the Python virtualenv build into `mkBlitzApi { pkgs, python ?
null }` and expose it as `lib.mkBlitzApi`. Rewrite `overlays.default`
to call the builder against the consumer's `prev` instead of pulling
from `self.packages.${system}` (which captured this flake's pinned
nixpkgs at flake-eval time and shut consumer overlays out of the
build hook).

Motivation: NixBlitz pulls blitz-api's flake via `builtins.getFlake`
to keep the plugin self-contained. Without a pkgs-agnostic builder,
the operator's nixpkgs overlays (Pi 5 jemalloc page-size patch and
similar platform fixes) couldn't reach blitz-api's build, so the
package would either fail to compile on Pi 5 or silently link
against this flake's pinned nixpkgs and pull a second snapshot of
glibc / openssl / etc. into the operator's closure.

The `packages.${system}.${name}` attribute is preserved as a
convenience pre-built using this flake's pinned nixpkgs; consumers
that want consumer-pkgs builds should use `overlays.default` or
`lib.mkBlitzApi` directly.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: fusion44

flake.nix

@@ -35,54 +35,80 @@
   }: let
     name = "blitz-api";
 
-    # Load the uv workspace once; reused across systems.
+    # Load the uv workspace once. Pure Nix value — independent of
+    # the consumer's pkgs, only depends on this repo's source tree.
     workspace = uv2nix.lib.workspace.loadWorkspace {workspaceRoot = ./.;};
 
-    # Pure Nix overlay derived from pyproject.toml + uv.lock. Prefer
-    # prebuilt wheels — a handful of deps (grpcio, psutil, pyzmq) are
-    # painful to compile from source and wheels work fine on linux/darwin.
-    overlay = workspace.mkPyprojectOverlay {
+    # Pyproject-nix overlay derived from pyproject.toml + uv.lock.
+    # Prefer prebuilt wheels — a handful of deps (grpcio, psutil,
+    # pyzmq) are painful to compile from source and wheels work
+    # fine on linux/darwin. Also pkgs-independent.
+    pyprojectOverlay = workspace.mkPyprojectOverlay {
       sourcePreference = "wheel";
     };
 
+    # Build the blitz-api Python virtualenv against the CALLER's
+    # pkgs. Exposed as `lib.mkBlitzApi` so consumers (NixBlitz
+    # plugin, downstream embedders) can build the package using
+    # their own nixpkgs — important when the consumer needs a
+    # platform-specific patch that this flake's pinned
+    # nixos-unstable doesn't carry yet.
+    #
+    # Without this, the package would be built once at THIS flake's
+    # eval time using THIS flake's pinned nixpkgs, and consumer
+    # overlays couldn't reach the build hook.
+    mkBlitzApi = {
+      pkgs,
+      python ? null,
+    }: let
+      py =
+        if python != null
+        then python
+        else pkgs.python312;
+      pythonSet = (pkgs.callPackage pyproject-nix.build.packages {python = py;})
+        .overrideScope (
+        pkgs.lib.composeManyExtensions [
+          pyproject-build-systems.overlays.default
+          pyprojectOverlay
+        ]
+      );
+    in
+      pythonSet.mkVirtualEnv "${name}-env" workspace.deps.default;
+
+    # Library output — what callers reach for to build the package
+    # under their own pkgs.
+    libOutput = {
+      lib = {inherit mkBlitzApi;};
+    };
+
     systems = flake-utils.lib.eachDefaultSystem (system: let
       pkgs = nixpkgs.legacyPackages.${system};
-      python = pkgs.python312;
-
-      pythonSet =
-        (pkgs.callPackage pyproject-nix.build.packages {
-          inherit python;
-        })
-        .overrideScope (
-          nixpkgs.lib.composeManyExtensions [
-            pyproject-build-systems.overlays.default
-            overlay
-          ]
-        );
     in {
       packages = {
         default = self.packages.${system}.${name};
-        ${name} = pythonSet.mkVirtualEnv "${name}-env" workspace.deps.default;
+        # Convenience: a pre-built package using THIS flake's pinned
+        # nixpkgs. Most consumers should use `overlays.default` or
+        # `lib.mkBlitzApi` instead so the package builds against
+        # their pkgs.
+        ${name} = mkBlitzApi {inherit pkgs;};
       };
     });
 
     overlays.overlays = {
+      # Re-runs `mkBlitzApi` against the consumer's `prev` pkgs so the
+      # consumer's overlays (Pi 5 page-size patch, custom uv pin, etc.)
+      # are in scope when the package is built.
       default = final: prev: {
-        ${name} = self.packages.${prev.stdenv.hostPlatform.system}.${name};
+        ${name} = mkBlitzApi {pkgs = prev;};
       };
     };
 
     module = {
-      nixosModules.default = {
-        pkgs,
-        lib,
-        config,
-        ...
-      }: {
+      nixosModules.default = {...}: {
         imports = [./modules/blitz_api.nix];
         nixpkgs.overlays = [self.overlays.default];
       };
     };
   in
-    systems // overlays // module;
+    systems // overlays // module // libOutput;
 }