feat(homepage): Australis Skyfall theme + Arbo-generated aurora background

Replaces the previous theme attempt, which was built on a misread: the ask was
to use Arbo as an IMAGE-GEN ENGINE for the background, with the operator's
Australis Skyfall design system supplying the palette.

theme/ holds the source — colors/layout/typography vendored verbatim from the
Skyfall handoff bundle, Supreme 400/500/700 woff2, the Homepage bindings in
skyfall.css.in, and build.py which inlines fonts + tokens into
conf/custom.css. custom.css is GENERATED; edit the .in file and rebuild.

The build exists because Homepage serves only custom.css and custom.js out of
its config dir, so a @font-face pointing at a vendored woff2 would 404 — the
face has to arrive as a data: URI. The background image takes the other route:
/app/public/images is a real static route, so compose.yaml now mounts
images/ there read-only and settings.yaml points at /images/.

Bindings map Skyfall's semantic layer onto Homepage's DOM: Sea surfaces, the
depth recipe (hairline AND two-layer shadow, never one alone), uppercase
eyebrow group headers, the sanctioned accent-rail on the active tab rather
than a glow, and semantic status colour so a green pill means the service is
actually serving.

Background generated by Arbo (irv-ml1:8201) workflow t2i-ui-background, job
13f0891f4e42, seed 26, flux2-klein-9b, 2048x1152 — abstract, no subject,
cool-temperature aurora. 1.6 MB PNG -> 22 KB WebP.

Two deviations are documented rather than hidden: Skyfall forbids imagery
behind body text (held at opacity 30 as mitigation), and service icons stay
full-colour vendor logos.

NOT DEPLOYED — live still runs the old theme. Prototype on :5199.

stacks/homepage/README.md

@@ -146,22 +146,78 @@ render are different questions.
 Before/after evidence: `~/booth-data/homepage-cleanup/` on nh3-dev →
 `http://10.100.10.50:8090/b/homepage-cleanup/` (24h TTL).
 
-## Theme — the Arbo "Raven" palette (`conf/custom.css`)
+## Theme — Australis Skyfall
 
-`conf/custom.css` carries a port of Arbo's design tokens (irv-ml1:8201): flat
-raven ink `#021425`, card surface `#112333` on `#1B2E3D` borders, Manrope, and
-mint `#2FFC89` reserved for signal — a status pill is green only when the
-service is actually serving. Values were read off Arbo's running `:root`
-custom properties, not sampled from a screenshot, so drift checks are a diff.
+The dashboard is themed with **Australis Skyfall**, the operator-supplied
+design system (handoff bundle, 2026-08-18). Skyfall is a dual-theme web system
+built on the OKLCH rebuild of the Australis terminal palette: one lightness law
+across every chromatic family, all hues cooler than neutral, a Sea neutral ramp
+that drifts from ice-blue to ocean green as it brightens, and a "calm depth"
+language of hairline + two-layer shadow on every elevated surface.
 
-It lives in CSS rather than `settings.yaml` because Homepage's `color:` setting
-only accepts built-in Tailwind ramps and has no separate surface/border/accent
-knobs. `color: slate` stays as the base ramp. The `!important` flags are
-load-bearing against Tailwind utility specificity.
+### `theme/` is the source; `conf/custom.css` is generated
 
-Pairs with removing the `background:` block from `settings.yaml` — Arbo has no
-background image, and the wallpaper fought the card surfaces, which sit only a
-little lighter than the page.
+```
+theme/colors.css      vendored VERBATIM from the bundle
+theme/layout.css      ditto — radii, the shadow/glow/rail system, motion
+theme/typography.css  ditto — families, scale, weights, tracking
+theme/fonts/          Supreme 400/500/700 woff2 (Fontshare ITF license)
+theme/skyfall.css.in  the Homepage bindings — the only part that is ours
+theme/build.py        inlines the fonts + tokens -> conf/custom.css
+```
+
+**Do not hand-edit `conf/custom.css`.** Change `skyfall.css.in`, run
+`python3 stacks/homepage/theme/build.py`, then deploy.
+
+The build exists for one reason: Homepage serves exactly two files out of its
+config directory, `custom.css` and `custom.js`. There is no static route for
+anything beside them, so a `@font-face` pointing at a vendored `.woff2` would
+404 — the face has to arrive inside the stylesheet as a data: URI. That is also
+why the background image is mounted at `/app/public/images` instead (see
+`compose.yaml`), which *is* a real static route.
+
+Only Supreme (body/UI) is embedded. A link dashboard has no display type, so
+Bespoke Sans buys nothing; Victor Mono ships as 2.4 MB TTF statics per cut,
+which is 30x the whole stylesheet for the handful of latency figures on the
+cards. Both are named in the font stacks so they resolve if vendored later.
+
+### The background is generated, not stock
+
+`images/skyfall-aurora.webp` came out of **Arbo** (irv-ml1:8201) via its
+`t2i-ui-background` workflow — "abstract full-bleed backgrounds, no subject".
+Job `13f0891f4e42`, seed 26, flux2-klein-9b, 2048x1152, 1.6 MB PNG re-encoded
+to a 22 KB WebP (smooth gradients compress absurdly well).
+
+Regenerate with a different seed, or reword, via:
+
+```bash
+curl -s -X POST http://10.100.79.3:8201/workflows/t2i-ui-background/run \
+  -H 'content-type: application/json' \
+  -d '{"prompt":{"kind":"raw","text":"...","negative":"..."},
+       "width":2048,"height":1152,"seed":26}'
+# poll GET /jobs/<id>, then GET /artifacts/<id>/0
+```
+
+Note the prompt field is a discriminated union, not a string — a bare string
+gets a 422. `{"kind":"raw","text":...}` is the shape.
+
+### Two deviations from the system, on the record
+
+1. **Skyfall forbids this background.** Its rule is "flat semantic surfaces; no
+   photography, no textures", with exactly one permitted decorative motif — a
+   subtle aurora gradient, on hero and empty-state areas only, *"never behind
+   body text blocks"*. A dashboard is a body-text block. The background is here
+   on the operator's explicit instruction to generate one with Arbo, and it is
+   mitigated rather than excused: abstract with no subject, strictly cool
+   temperature per the imagery rule, and held at `opacity: 14` so card contrast
+   never pays for it. **That opacity is load-bearing, not decoration** — the
+   render is brighter than the "mostly empty negative space" brief, and turning
+   it up is what would make the cards fight the ribbon. If it ever reads busy,
+   the system-faithful answer is to drop the image and let `--surface-sunken`
+   be the canvas.
+2. **Service icons stay full-colour vendor logos.** Skyfall wants one cool,
+   technical icon family. Desaturating every logo from CSS would just make them
+   illegible, so this one is simply not fixed.
 
 ## Open question