Optimize pureintent eval time (Ralph) (#117)

* docs: Ralph report scaffolding + pureintent eval baseline (10.87s)

* cycle-1: drop nixvim — pureintent eval 10.87s → 6.98s (-36%)

nixvim's home-manager module system is by far the dominant cost of
evaluating `nixosConfigurations.pureintent`: a profile of selective
imports puts it at 3.97s of the 10.87s baseline (~36%).

Replace it with a minimal `programs.neovim.enable = true` configuration.
The runtime behaviour of `nvim` changes (no plugins, no LSP, no
mapleader/telescope/treesitter/etc.) — accepted by the user.

Measured on srid-nc (nixos, 6.12.85 kernel), 7 warm-cache runs with
`--option eval-cache false`:

  baseline: 10.85 10.86 10.86 10.87 10.87 10.88 10.91 -> median 10.87s
  cycle-1:  6.94  6.96  6.98  6.98  7.02  7.02  7.73 -> median 6.98s

Other configurations still evaluate:
  - nixosConfigurations.naiveintent
  - darwinConfigurations.infinitude-macos
  - homeConfigurations."srid@zest"

Removes flake input `nixvim` (+ its `flake-parts`, `nixpkgs`, `systems`
sub-inputs from the lock).

* docs: wrap up Ralph report (10.87s -> 6.99s = -35.7%)

7 dead-ends documented so they don't have to be re-tried. Key
finding: after dropping nixvim (cycle 1), the eval floor is
home-manager's per-entry submodule materialisation
(systemd.user.services / programs.ssh.matchBlocks), not the
option-declaration count of any wrapper module. Inlining
jumphost-nix or vira moves the cost; it does not eliminate it.

modules/flake-parts/nixos-flake.nix

@@ -16,7 +16,6 @@
         "nix-darwin"
         # "nixos-hardware"
         "nix-index-database"
-        "nixvim"
       ];
     };
   };