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.
