Ralph: pureintent eval-time optimization

Iterative measurement-driven shrinking of nixosConfigurations.pureintent evaluation time, following the Ralph skill.

Methodology

Host: srid-nc (NixOS, x86_64-linux, Nix 2.31.5)
Repo: cloned at ~/ralph/nixos-config, branch optimize-eval
Command:
```
/usr/bin/env time -f "%e" \
  nixos-rebuild dry-build --flake .#pureintent \
                --option eval-cache false
```
- dry-build so no derivations are actually realised.
- --option eval-cache false because every meaningful source change invalidates the flake eval-cache — measuring with a hot eval-cache just measures sqlite lookups (~0.5 s) and tells us nothing about the work this PR is meant to cut.
- Filesystem / Nix-store cache is warm (we ran the build once to populate everything).
Baseline policy: 5–7 consecutive runs, report median, range, and NIX_SHOW_STATS counters (nrFunctionCalls, nrThunks, totalBytes, etc.).
Noise floor: ≈0.5 % (range was 0.06 s on a 10.87 s run). Commit threshold per Ralph rules: > 3 %.
Constraints: (1) other NixOS / home configs must still build, (2) pureintent runtime behaviour preserved (drv hash may shift), (3) reducing flake inputs is in scope (npins-style relocation OK).

Baseline

7 warm runs, eval-cache disabled, nixos-rebuild dry-build .#pureintent:

run	seconds
1	10.87
2	10.87
3	10.86
4	10.88
5	10.86
6	10.85
7	10.91

Median: 10.87 s · range 0.06 s (≈ 0.5 %).

NIX_SHOW_STATS for one baseline eval:

counter	value
CPU time	16.29 s
GC fraction	3.3 %
`nrFunctionCalls`	25 043 557
`nrPrimOpCalls`	12 110 404
`nrThunks`	38 038 010
`nrAvoided`	32 199 960
`nrLookups`	16 259 247
`nrOpUpdates`	2 469 935
`nrOpUpdateValuesCopied`	85 701 830
`values.number`	57 025 753
`values.bytes`	912 412 048
`sets.elements`	120 709 352
`sets.bytes`	2 059 750 432
`totalBytes`	4 139 856 480
`maxRss`	3 120 MB

Flake input inventory

Inputs declared in flake.nix:

agenix, disc-scrape, disko, emanote, flake-parts, git-hooks, github-nix-ci, home-manager, imako, jumphost-nix, kolu, landrun-nix, llm-agents, nix-darwin, nix-index-database, nixos-hardware, nixos-unified, nixos-vscode-server, nixpkgs, nixvim, project-unknown, vira, zmx.

Per-input blame is filled in as cycles run.

Optimization log

#	hypothesis	mutation	wall (s)	Δ vs baseline	committed?	notes
0	—	baseline	10.87	—	—	reference

Dead ends

(none yet)

Key findings

(filled in at wrap-up)

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