build: codec presets for smaller core builds #51

Part of #49. Idea from wasmpeg's data-driven presets (https://github.com/wasmpeg/wasmpeg, src/cli/configure.mjs and docs/formats/codecs.md). Their core is about 3.2 MB gzipped against our roughly 10.2 MB st core, mostly from codec and tool selection.

Add a documented preset list (for example full, the current default, and decode, which keeps decoders plus a small encoder set) that drives the configure flags in the Dockerfile. The published packages keep full, so nothing breaks. Users building their own core can pick a smaller preset. Record the size of each preset in the docs.

Serves the "small bundle size and low memory use" direction in AGENTS.md.