Add project icon

A broken method chain with a column guide: the formatter's own output shape,
rather than a generic "order" or "building" metaphor. A brick-wall idea was
considered and dropped — a wall reads as a barrier, which is the opposite of
what this fork is for, and a uniform grid of identical blocks advertises the
mechanical layout this formatter deliberately does not do.

The boundary is carried by two bars sharing a right edge rather than by a
drawn line, so it survives 16px where a hairline turns to mush. The dashed
guide is a second reading for larger sizes only.

docs/icon/README.md records the geometry that must not be broken, because
every variant of this looked fine at 64px and only the small size rejects
anything.

Author: abashev

docs/icon/README.md

@@ -0,0 +1,38 @@
+# Icon
+
+A broken method chain: one long line, then a group of shorter ones sharing a left edge, with a
+column guide down the right. It is the formatter's own output shape — the thing that distinguishes
+it is deciding *where* to break and what to line up, not laying everything out uniformly.
+
+| File | Use |
+|---|---|
+| `icon.svg` | The mark. Inherits `currentColor`, so one file covers light, dark and monochrome. |
+| `favicon.svg` | Same geometry, guide at full strength. Below ~24px contrast is what keeps the guide visible, not shape. |
+| `avatar.svg` | The mark on a white rounded square, for the GitHub organisation and marketplace tiles. |
+| `avatar-512.png`, `avatar-128.png` | Rasterised from `avatar.svg`; GitHub organisation avatars must be raster. |
+
+Regenerate the PNGs after editing the SVG:
+
+```bash
+rsvg-convert -w 512 -h 512 docs/icon/avatar.svg -o docs/icon/avatar-512.png
+rsvg-convert -w 128 -h 128 docs/icon/avatar.svg -o docs/icon/avatar-128.png
+```
+
+## What not to break
+
+The mark has seven elements, which is the ceiling for 16px. Anything added has to displace
+something else.
+
+- **The shared right edge does the work.** The first and third bars end on the same x. That
+  implied vertical is what carries "there is a boundary" at sizes where the dashes disappear, and
+  it costs nothing because no thin element draws it. Changing either bar's width breaks the idea
+  silently — the icon will still look fine at 64px and mean nothing at 16px.
+- **The guide's rhythm must not match the bars'.** Three dashes against four bars is deliberate:
+  mismatched rhythms read as two layers, so the boundary belongs to the format rather than to any
+  one line. Align them and the dashes turn into decoration stuck on the bar ends.
+- **Check every edit at 16px first.** Every version of this mark looked fine large. The small size
+  is the only one that rejects anything.
+
+The two colours in `avatar.svg` are the only brand decision baked in here, and they are a
+placeholder. The white ground means that on GitHub's light theme the tile itself disappears and
+only the mark reads; an off-white ground keeps the tile visible if that is preferred.