GoAT WebAssembly build

This is a narrow host wrapper around upstream github.com/blampe/goat v0.5.1. The upstream parser, ASCII/UTF-8 geometry, SVG writer, and embedded CSS remain the source of truth. TinyGo is used only to produce a Workers-compatible wasm-unknown module; the browser-facing adapter owns request validation and copies the returned SVG from linear memory. It calls beginRender before allocating source/options; the wrapper retains those buffers through the render call so TinyGo's garbage collector cannot invalidate host-owned pointers.

Build from Docker with the pinned TinyGo image in provenance.md:

docker run --rm -e GOFLAGS='-buildvcs=false -mod=vendor' \
  -v "$PWD:/src" -w /src/renderers/edge/toolchains/goat \
  tinygo/tinygo:0.39.0 \
  tinygo build -scheduler=none -target wasm-unknown \
    -o ../../artifacts/goat/goat.wasm .