Add brand assets and put the logo at the top of the README

The mark is an open arc — the bounded ceiling a governed run executes under,
and literally the ARC — wrapping a three-node graph, the Graph. One shape for
both halves of the name; the gap at the top keeps it from reading as a
generic ring.

Wordmark is Poppins, Medium for "Graph" and Bold for "ARC" so the gradient
lands on the emphasized half. The type is converted to vector outlines rather
than <text> elements, so the SVGs render identically without Poppins
installed anywhere.

README uses <picture> with prefers-color-scheme, so GitHub serves the
light-on-dark lockup in dark mode and the dark-on-light one in light mode,
both transparent. The <img> fallback is the light asset for clients that
ignore the source elements.

docs/brand/grapharc-favicon.svg is deliberately not the icon scaled down: it
carries heavier strokes and larger nodes because the standard icon's inner
triangle turns to mush below ~48px. Rasters at 16/32/64/180/512.

One implementation note, recorded in docs/brand/README.md because it is a
silent failure mode: the gradients use gradientUnits="userSpaceOnUse". Under
the default objectBoundingBox, a perfectly horizontal stroke has a
zero-height bounding box, which leaves the gradient undefined and the element
simply does not paint — the triangle's top edge disappeared from the first
render for exactly that reason, with no error anywhere.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: claude

README.md

@@ -1,3 +1,11 @@
+<p align="center">
+  <picture>
+    <source media="(prefers-color-scheme: dark)" srcset="docs/brand/grapharc-logo-ondark.svg">
+    <source media="(prefers-color-scheme: light)" srcset="docs/brand/grapharc-logo-light.svg">
+    <img alt="GraphARC" src="docs/brand/grapharc-logo-light.svg" width="440">
+  </picture>
+</p>
+
 # GraphARC
 
 [![PyPI](https://img.shields.io/pypi/v/grapharc.svg)](https://pypi.org/project/grapharc/)

docs/brand/README.md

@@ -0,0 +1,53 @@
+# GraphARC brand assets
+
+The mark is an open arc — the bounded ceiling a governed run executes under, and
+literally the *ARC* — wrapping a three-node graph, the *Graph*. One shape for
+both halves of the name; the gap at the top keeps it from reading as a generic
+ring.
+
+Wordmark is Poppins (Medium for `Graph`, Bold for `ARC`, so the gradient lands
+on the emphasized half). **The type is converted to vector outlines, not `<text>`
+elements**, so every SVG here renders identically without Poppins installed.
+
+## Which file to use
+
+| File | Use for |
+| --- | --- |
+| `grapharc-logo-ondark.svg` | Lockup on a dark background (transparent) — this is the README's dark-mode asset |
+| `grapharc-logo-light.svg` | Lockup on a light background (transparent) |
+| `grapharc-logo-dark.svg` | Lockup with its own dark rounded panel, for placing on an arbitrary background |
+| `grapharc-icon-transparent.svg` | Icon alone, no background |
+| `grapharc-icon-dark.svg` | Icon on a dark rounded square — social avatars, app icons |
+| `grapharc-favicon.svg` + `grapharc-favicon-*.png` | Small sizes only (see below) |
+
+`.png` siblings are 3x raster exports of the same artwork, for contexts that
+cannot take SVG. Prefer the SVG wherever it is accepted.
+
+## Use the favicon file below ~48px
+
+`grapharc-favicon.svg` is not the same artwork scaled down — it carries heavier
+arc and edge strokes and larger nodes, because the standard icon's inner
+triangle turns to mush at favicon sizes. Rasters are provided at 16, 32, 64, 180
+(Apple touch icon) and 512.
+
+## Palette
+
+The gradient runs teal → blue → violet:
+
+| Stop | Icon arc | Inner graph (lighter, for contrast against the arc) |
+| --- | --- | --- |
+| 0% | `#2DD4BF` | `#5EEAD4` |
+| 50–55% | `#3B82F6` | `#60A5FA` |
+| 100% | `#8B5CF6` | `#A78BFA` |
+
+Dark background: `#0B0F19`. Wordmark: `#E7ECF3` on dark, `#0F172A` on light.
+
+## If you edit these
+
+Gradients use `gradientUnits="userSpaceOnUse"`, and that is deliberate rather
+than incidental. Under the default `objectBoundingBox`, a perfectly horizontal
+or vertical stroke has a zero-height (or zero-width) bounding box, which makes
+the gradient undefined — so the element silently does not paint at all. The
+triangle's top edge is exactly that case, and it vanished from the first render
+of this mark for that reason. Keep the user-space units, or that edge disappears
+again with no error anywhere.