Samplers: emblem at original 8rem size, brand/nav centre-aligned

Review feedback: the emblem looked bad too small, and the brand text
sat out of line with the navbar. All three directions now render the
emblem at the app's real height: 8rem with the -0.2rem margin-block
trim, from a 256px 64-colour quantised data URI so it stays crisp.

Alignment fixed by centre-aligning the header row (the old baseline
alignment against a tall inline image is what pushed the text out);
verified with getBoundingClientRect that brand, emblem, nav and
summary share one vertical mid in every direction (Primer 75.2,
Control 72.0, Swiss 68.0). Swiss's open-group red rule moved to an
absolutely positioned ::before so the indicator no longer shifts the
summary box.

mocks/README.md

@@ -17,10 +17,15 @@ is purely visual.
 ## Header
 
 Every direction keeps the real app's chrome: the emblem + ClankerSaddle
-brand and the three nav groups from `main.html` — Work (Dashboard,
-Tasks, Goals, Chats), Library (Projects, Rules, Stacks, Discoveries),
-Admin (Saddles, Users, Model) — plus the Account menu. Pure CSS
-`<details>` dropdowns, no JS.
+brand at the app's original `height: 8rem` (with its `-0.2rem`
+margin-block trim), and the three nav groups from `main.html` — Work
+(Dashboard, Tasks, Goals, Chats), Library (Projects, Rules, Stacks,
+Discoveries), Admin (Saddles, Users, Model) — plus the Account menu.
+Pure CSS `<details>` dropdowns, no JS. Brand and nav are centre-aligned
+on one axis (verified via engine geometry: brand/emblem/nav/summary
+mids equal to 0.1px in all three directions). The emblem is embedded
+per file as a 256px, 64-colour-quantised PNG data URI (~31 KB base64)
+so the samplers stay standalone while rendering crisp at 8rem.
 
 ## Content model (per the approved hierarchy)
 
@@ -138,5 +143,7 @@ rationed.
   content extents identical between schemes (no reflow breakage).
 - The three `<body>` blocks verified identical modulo sampler label.
 - Nav dropdowns verified open (per-direction probe renders); emblem
-  verified visible in both schemes in all three directions.
+  verified visible in both schemes in all three directions, rendered
+  at the app's original 8rem height; brand/nav vertical alignment
+  verified equal via getBoundingClientRect mids.
 - 390px mobile: no horizontal overflow; all zones render.