docs: add maintainer build guide

[rauhryan]

Summary

• add a maintainer-focused BUILD.md for local source builds
• link README.md to the new build guide
• document submodule setup, toolchain setup, rebuild triggers, verification, and troubleshooting

Verification

• followed the documented build flow locally
• verified Homebrew LLVM clang can target wasm32
• ran git submodule update --init --recursive
• ran make clean && make
• ran deno task test

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/clayterm@28

commit: a44b553