feat: adopt the new Helmryth mark

brand/README.md

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+# Brand assets
+
+Source of truth for the Helmryth mark. Everything here is SVG; the raster icons
+under `build/` are generated from `helmryth-tile.svg` by `pnpm build:icons` and
+should never be edited by hand.
+
+| File | Use |
+|---|---|
+| `helmryth-mark.svg` | The mark alone, transparent. Web, favicons, anywhere a bare glyph is wanted. |
+| `helmryth-mark-mono.svg` | Same geometry, `currentColor`. Inherits the surrounding text colour. |
+| `helmryth-tile.svg` | The app tile — mark on warm paper inside the clipped octagon. Master for every generated icon. |
+| `helmryth-lockup.svg` | Mark and wordmark, for headers and READMEs. |
+
+## The mark
+
+A run line, halted at a notch, with a hand-set arm seated in it.
+
+The thin petrol rule is work running. The heavy vermilion arm is a person set into
+that run at an angle nothing automatic would have chosen. The notch holds the
+setting so it cannot drift. It is the product's one claim drawn as a shape:
+nothing runs until you answer.
+
+## Colour
+
+| Token | Hex | Meaning |
+|---|---|---|
+| Vermilion | `#C44F36` | Command — the human act. Never decorative. |
+| Petrol | `#167A78` | Live signal — work in motion. |
+| Paper | `#F6F1E7` | Surface. |
+| Rule | `#D6CAB9` | Structure. |
+| Ink | `#273447` | Type. |
+
+That vermilion/petrol split is load-bearing: the application uses the same two
+colours for the same two meanings. A mark that swapped them would fight the UI.
+
+## Rules
+
+The mark must read in one colour before it reads in two — use
+`helmryth-mark-mono.svg` to check. Do not recolour it, rotate it, add a glow or a
+gradient, place it on a busy photograph, or set the wordmark in a different
+typeface. The wordmark is Atkinson Hyperlegible Next, the same face the product
+uses, chosen because legibility is a product principle rather than a style.