nix: hyperhive owns nixpkgs pin via follows; bump to nixos-26.05 (#526)

Drops the hardcoded `nixpkgs.url = "github:NixOS/nixpkgs/nixos-25.11"`
from the meta flake renderer. meta now declares:

    nixpkgs.follows         = "hyperhive/nixpkgs";
    nixpkgs-unstable.follows = "hyperhive/nixpkgs-unstable";

so every agent-level `inputs.<X>.inputs.nixpkgs.follows = "nixpkgs"`
resolves transitively to hyperhive's own pin. One channel decision
in the whole tree (hyperhive/flake.nix line 5), no second source
to drift.

`closes #317` invariant still holds: `nixpkgs` is a single canonical
name in the meta tree, it just resolves through hyperhive instead
of being its own root input.

Also:
- bump hyperhive/flake.nix line 5: `nixos-25.11` → `nixos-26.05`,
  flake.lock relocked
- flake_check.rs fixtures bumped cosmetically (synthetic data, not
  shape-affecting)
- rename + rewrite `render_flake_declares_canonical_nixpkgs` →
  `render_flake_aliases_nixpkgs_to_hyperhive`; asserts the new
  follows-form AND the absence of any literal `nixpkgs.url`
- drop the now-redundant `render_flake_collapses_hyperhive_nixpkgs_via_follows`
  test (the old indirection it covered no longer exists)

Closes #526.

flake.lock

@@ -17,16 +17,16 @@
     },
     "nixpkgs": {
       "locked": {
-        "lastModified": 1778737229,
-        "narHash": "sha256-6xWoytx8jFW4PF1GjRm/i/53trbpKGfz6zjzQGBr4cI=",
+        "lastModified": 1779971959,
+        "narHash": "sha256-R5nauXyqyfRUFiZycFFZdkF7wl6eaUpPLst35+2nJQY=",
         "owner": "NixOS",
         "repo": "nixpkgs",
-        "rev": "d7a713c0b7e47c908258e71cba7a2d77cc8d71d5",
+        "rev": "ec942ba042dad5ef097e2ef3a3effc034241f011",
         "type": "github"
       },
       "original": {
         "owner": "NixOS",
-        "ref": "nixos-25.11",
+        "ref": "nixos-26.05",
         "repo": "nixpkgs",
         "type": "github"
       }