feat(flake): #134 — unstable.<pkg> in the downstream, one input, no hand pins

Nomarchy now carries a nixpkgs-unstable input and exposes it through
overlays.default as an `unstable` attrset: in the template's
`home.packages = with pkgs; [ … ]` a user writes `unstable.lmstudio`
and is done. Downstream stays one locked input; nomarchy-pull moves it.
lib.nix needed zero changes — mkFlake's pkgs already applies the
overlay. The import is plain and non-recursive, so configs that never
reference unstable. pay nothing beyond the input fetch; allowUnfree
mirrors the base set so the motivating lmstudio resolves.

Home packages are the supported surface (documented convention). Costs
stated in the README, not hidden: you own the pinned+unstable
combination; a second toolchain lands in your closure (~690 of ~4k
paths shared, measured on lmstudio); it moves when the lock moves.
Lock bumps now move two channels — VERIFICATION §5 and CONVENTIONS
updated accordingly. flake.lock gained exactly one node
(nixos-unstable @ 753cc8a); nothing else moved.

Verification: V1 — nix flake check --no-build green; new eval-only
checks.unstable-seam green (26.05pre vs 26.11pre lib.version, no
builds); template HM activation built via mkFlake with unstable.hello
enabled and its closure carries the unstable store path (pg2zfr… vs
pinned nm7p8w…). Implementation by a Sonnet subagent; design and
review on Fable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

flake.nix

@@ -4,6 +4,11 @@
   inputs = {
     nixpkgs.url = "github:NixOS/nixpkgs/nixos-26.05";
 
+    # Second, newer channel — exists solely to feed the `unstable.*` overlay
+    # seam (#134), so downstream can pick individual newer apps without a
+    # second flake input of their own.
+    nixpkgs-unstable.url = "github:NixOS/nixpkgs/nixos-unstable";
+
     home-manager = {
       url = "github:nix-community/home-manager/release-26.05";
       inputs.nixpkgs.follows = "nixpkgs";
@@ -27,7 +32,7 @@
     };
   };
 
-  outputs = { self, nixpkgs, home-manager, stylix, nixos-hardware, ... }:
+  outputs = { self, nixpkgs, nixpkgs-unstable, home-manager, stylix, nixos-hardware, ... }:
     let
       system = "x86_64-linux";
       username = "nomarchy"; # reference host only; downstream picks its own
@@ -79,6 +84,17 @@
       # the `nomarchy.stateFile` option — reading it stays pure.
 
       overlays.default = final: prev: {
+        # A second, newer channel for individual apps (#134) — e.g. lmstudio,
+        # whose fast-moving upstream a pinned nixpkgs lags. Plain `import`,
+        # deliberately NOT final-recursive: it evaluates only when a config
+        # actually references `unstable.something`, so checkouts that never
+        # write `unstable.` pay nothing for it. allowUnfree mirrors the base
+        # set (lib.nix) so unfree apps — lmstudio is the motivating case —
+        # resolve here too.
+        unstable = import nixpkgs-unstable {
+          inherit (prev.stdenv.hostPlatform) system;
+          config.allowUnfree = true;
+        };
         nomarchy-state-sync = final.callPackage ./pkgs/nomarchy-state-sync {
           # Presets baked into the package, so `nomarchy-state-sync list`
           # works on machines that don't check out this repo.
@@ -2268,6 +2284,22 @@
               )
             '';
           };
+
+          # #134: proves the `unstable.*` overlay seam exists and is really a
+          # second channel, without building anything from it — evaluation-only,
+          # so this stays cheap even though nixpkgs-unstable is a whole extra
+          # input. `pkgs` here already carries overlays.default (line ~35).
+          unstable-seam =
+            let
+              expect = cond: msg: nixpkgs.lib.assertMsg cond "unstable-seam: ${msg}";
+              ok =
+                expect (pkgs.unstable ? hello)
+                  "pkgs.unstable has no `hello` — the overlay attr is missing or broken"
+                && expect (pkgs.unstable.lib.version != pkgs.lib.version)
+                  ("pkgs.unstable.lib.version == pkgs.lib.version — this is the same "
+                    + "channel as the pinned nixpkgs, not a second one");
+            in
+            assert ok; pkgs.runCommand "nomarchy-unstable-seam" { } "touch $out";
         };
 
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