docs(roadmap): record the faster-switches diagnosis + decided approach

Flesh out the "Faster switches" item with the confirmed diagnosis (themes/
is 86 MB and backgrounds/ is all of it; the config is ~208 KB; wallpapers
are never read at eval; theme-state.json is git-tracked so every apply
re-copies the whole 86 MB source) and the decided-but-deferred approach:
Option 1, a separate pinned wallpapers artifact (repo/tarball via flake
input/fetchurl, read through the existing NOMARCHY_DEFAULT_THEMES hook),
which keeps eval pure. Notes the rejected impure-state alternative and the
pre-built-theme-variants follow-on.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

docs/ROADMAP.md

@@ -107,9 +107,26 @@ how to override it. Items marked ✓ are shipped.
   rendered + captured); real-hardware capture was chosen for fidelity +
   simplicity, with the VM route as a documented fallback if hand-capturing all
   themes gets tedious.)
-- **Faster switches:** move `backgrounds/` out of the flake source (the 86 MB
-  re-copy on every state write is the main eval tax), then pre-built theme
-  variants if still needed
+- **Faster switches:** move `backgrounds/` out of the flake source. Diagnosis
+  (confirmed): `themes/` is 86 MB and `backgrounds/` is **all** of it — the
+  palette JSONs + per-theme overrides are only ~208 KB, and the wallpapers are
+  **never read at Nix eval** (only the Python `swww` path uses them). But
+  `theme-state.json` is git-tracked, so every `apply` rewrites it → the flake
+  tree changes → Nix re-copies the whole 86 MB source before `home-manager
+  switch` can evaluate. You pay an 86 MB copy to change a 1 KB file.
+  **Decided approach (deferred — don't want the extra moving part yet):**
+  Option 1, a **separate pinned wallpapers artifact** — a `Nomarchy-wallpapers`
+  repo or release tarball, pulled once via a flake input / `fetchurl` (pinned
+  by hash → content-addressed, never re-copied on a state write), with
+  `nomarchy-theme-sync` reading wallpapers from that stable store path (the
+  `NOMARCHY_DEFAULT_THEMES` env hook already anticipates external theme
+  assets). Keeps eval **pure**; the live ISO still bakes them in (fetched at
+  build). A state write then re-copies only ~208 KB. Rejected alternative:
+  moving `theme-state.json` out to `~/.config` + `--impure` eval (one repo, no
+  second artifact, but trades away the in-tree-pinned-state reproducibility).
+  Follow-on if `home-manager switch` itself is still the bottleneck after the
+  copy is gone: **pre-built theme variants** (build each theme's generation
+  ahead of time so a switch just activates a cached one).
 - Greeter (tuigreet/SDDM) theming from the same JSON (Plymouth ships since
   v1: `nomarchy.system.plymouth.*`, background tinted from the state file)
 - Installer round 2: multi-disk BTRFS RAID, impermanence, BIOS/legacy