ci: tag commit as stable after full test matrix

- add mark-stable workflow that runs on workflow_run for all test pipelines
- use GitHub API to ensure all required workflows succeeded before moving the 'stable' tag
- add Nix flake.lock to pin nixpkgs for reproducible builds

https://chatgpt.com/share/693aa4a6-7460-800f-ba47-cfc15b1b2236

flake.lock

@@ -0,0 +1,27 @@
+{
+  "nodes": {
+    "nixpkgs": {
+      "locked": {
+        "lastModified": 1765186076,
+        "narHash": "sha256-hM20uyap1a0M9d344I692r+ik4gTMyj60cQWO+hAYP8=",
+        "owner": "NixOS",
+        "repo": "nixpkgs",
+        "rev": "addf7cf5f383a3101ecfba091b98d0a1263dc9b8",
+        "type": "github"
+      },
+      "original": {
+        "owner": "NixOS",
+        "ref": "nixos-unstable",
+        "repo": "nixpkgs",
+        "type": "github"
+      }
+    },
+    "root": {
+      "inputs": {
+        "nixpkgs": "nixpkgs"
+      }
+    }
+  },
+  "root": "root",
+  "version": 7
+}