chore(flake): refresh nixpkgs inputs to 2026-09-11

Bump nixpkgs and nixpkgs-latest from 2026-05/2026-08 to current
nixos-unstable (2026-09-11); both inputs now resolve to the same rev.
Re-validated every package against the new inputs.

Fallout fixed:
- freetoken: upstream pins torch>=2.11,<2.12 and triton==3.6.0, but the new
  nixpkgs only ships torch 2.13 / triton 3.7. Add a nixpkgs-torch211 input
  pinned at the previous revision and build freetoken against it, keeping
  the CUDA 12.9 / torch 2.11 stack unchanged.
- graphify: nixpkgs' pybloomfilter3 0.7.3 ships sdist metadata that still
  says 0.7.2, tripping pythonMetadataCheckPhase; skip that check only.

All 22 packages build and `nix flake check --no-build` passes.

flake.nix

@@ -6,6 +6,11 @@
     # Newer nixpkgs whose toolchains are required by some packages:
     # rustc >= 1.95 (mcp-gateway >= 3.4.0), go >= 1.26.3 (kubernetes-mcp-server >= 0.0.66).
     nixpkgs-latest.url = "github:NixOS/nixpkgs/nixos-unstable";
+    # freetoken pins torch>=2.11,<2.12 and triton==3.6.0; current
+    # nixos-unstable only ships torch 2.13 / triton 3.7. Keep the previous
+    # nixpkgs revision for freetoken so its CUDA 12.9 / torch 2.11 stack is
+    # unchanged.
+    nixpkgs-torch211.url = "github:NixOS/nixpkgs/549bd84d6279f9852cae6225e372cc67fb91a4c1";
     systems.url = "github:nix-systems/default";
     blueprint = {
       url = "github:numtide/blueprint";