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

[Shashankss1205]

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 elements, so the SVGs render identically without Poppins installed anywhere.

README uses 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 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.