feat(theme): SVOS retheme — concept-round candidate

Re-skins every Booth surface in the SVOS design system (design-systems
palettes/svos @ ed2f8d8). Visual and interaction layer only: no route,
no copy, no ordering and no information-architecture change.

- _svos_tokens.css: SVOS semantic tokens vendored by copy, with the four
  [data-theme] scopes re-scoped onto prefers-color-scheme and
  prefers-contrast (dark, light, dark-hc, light-hc). Included into
  base.html's <style>; cached at startup like every other template.
- base.html: the accreted Australis sheet is rewritten against semantic
  tokens only. It also fixes four undefined variables (--line, --bg,
  --fg, --muted) that the keep/blur/reveal controls had been reading.
  The three SVOS devices each have exactly one job: reticle = selection
  (grid cursor, :target, picked option), hazard = irreversible (Wipe
  now, armed bulk delete), glow = live power (service dot, live bench).
- The flag list renders as wrapped chips, so a large flag set no longer
  pushes the grid below the fold. The list order is unchanged.
- IBM Plex Sans + JetBrains Mono load via Google Fonts with
  display=swap and system fallbacks (approved by booth-dev).
- view.html, doc.html: inline styles moved onto tokens.
- embed.js: fragment palette as custom properties scoped to .bk-ask;
  `.bk-ask-opt:has(input:checked)` still appears exactly once.
- Favicon (base.html + app.FAVICON_HREF, kept in sync): graphite tile
  with reticle corners.

Verified: 642 passed, the same count as the pre-change baseline.
Visual order matches document order on 32 renders (4 booths x 4 widths
x 2 schemes). A positive control, one tile given `order:-1`, is
detected by the same check.

booth/templates/view.html

@@ -47,17 +47,19 @@
 </div>
 <style>
   .vnav{position:fixed;top:50%;transform:translateY(-50%);z-index:40;display:flex;
-    align-items:center;justify-content:center;width:2.6rem;height:3.4rem;font-size:2rem;
-    line-height:1;text-decoration:none;color:var(--fg-1);background:rgba(20,23,32,.55);
-    border:1px solid rgba(255,255,255,.10);border-radius:10px;margin:0 .5rem;user-select:none;
-    -webkit-backdrop-filter:blur(4px);backdrop-filter:blur(4px);transition:background .15s,border-color .15s}
-  .vnav:hover{background:rgba(28,33,46,.92);border-color:var(--aus-bright-cyan,#42dcd1)}
+    align-items:center;justify-content:center;width:40px;height:56px;font-size:28px;
+    line-height:1;text-decoration:none;color:oklch(0.91 0.008 216);background:oklch(0.17 0.01 250 / .6);
+    border:1px solid rgb(255 255 255 / .12);border-radius:var(--radius-lg);margin:0 8px;user-select:none;
+    -webkit-backdrop-filter:blur(4px);backdrop-filter:blur(4px);
+    transition:background var(--dur-1) var(--ease-out),border-color var(--dur-1) var(--ease-out)}
+  .vnav:hover{background:oklch(0.21 0.01 248 / .92);border-color:rgb(255 255 255 / .3);text-decoration:none;
+    color:oklch(0.91 0.008 216)}
   .vprev{left:0}.vnext{right:0}
   /* Bottom bar rather than the top chrome: a caption can run to CAPTION_MAX
      (800 chars), which would shove the filename and the Fit/1:1 toggle around. */
-  .vcap{flex:0 0 auto;max-height:22vh;overflow-y:auto;padding:.6rem clamp(12px,3vw,20px);
-    font-size:.85rem;line-height:1.5;color:var(--fg-1);background:var(--rk-surface,rgba(20,23,32,.92));
-    border-top:1px solid rgba(255,255,255,.10);white-space:pre-wrap}
+  .vcap{flex:0 0 auto;max-height:22vh;overflow-y:auto;padding:10px clamp(12px,3vw,20px);
+    font-size:var(--size-body);line-height:var(--leading-body);color:var(--text-body);background:var(--surface-base);
+    border-top:1px solid var(--border-subtle);white-space:pre-wrap}
   @media print{.vcap{max-height:none;overflow:visible}}
   @media print{.vnav{display:none}}
 </style>