CI: split Nix CI into build/test/cross jobs with full platform matrix

Restructure nix-ci.yml from a monolithic build into separate jobs:
- Build: stage0+stage1 then stage2, with aggressive intermediate cleanup
- Test: download dist artifact and run testsuite standalone
- Cross: JS (emscripten) and WASM (wasi-sdk) backend builds

Full platform matrix: x86_64-linux (ubuntu-latest), aarch64-linux
(ubuntu-24.04-arm), aarch64-darwin (self-hosted), each with dynamic=0
and dynamic=1 configurations. Cross-builds run in parallel with tests
using continue-on-error.

Pass DYNAMIC to both stage1 and stage2 so configure generates consistent
cabal.project.stage2.settings (shared: True when dynamic). Always pass
USE_SYSTEM_CABAL=1 to stage2 since cabal was already built in stage1.
Add test timing file creation for metrics-test.svg generation.

Author: angerman