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.

hive-c0re/src/flake_check.rs

@@ -319,7 +319,7 @@ mod tests {
         "nodes": {
             "nixpkgs": {
                 "locked": {"rev": "aaa"},
-                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-25.11", "type": "github"}
+                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-26.05", "type": "github"}
             },
             "nixpkgs-unstable": {
                 "locked": {"rev": "bbb"},
@@ -335,15 +335,15 @@ mod tests {
         "nodes": {
             "nixpkgs": {
                 "locked": {"rev": "aaa"},
-                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-25.11", "type": "github"}
+                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-26.05", "type": "github"}
             },
             "nixpkgs_2": {
                 "locked": {"rev": "ccc"},
-                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-25.11", "type": "github"}
+                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-26.05", "type": "github"}
             },
             "nixpkgs_3": {
                 "locked": {"rev": "ddd"},
-                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-25.11", "type": "github"}
+                "original": {"owner": "NixOS", "repo": "nixpkgs", "ref": "nixos-26.05", "type": "github"}
             },
             "treefmt-nix": {
                 "locked": {"rev": "eee"},