Refactor QA layers

[kmolan]

What & why

Why:

The workspace has six overlapping QA layers — doctests (~112), tests/ (~24 files / ~6k lines / ~300 tests in 13 binaries), 15 examples/, criterion benches/ with hand-maintained .md accuracy tables, the tools/oracle golden-fixture crate, and the tools/embedded-smoke QEMU firmware — plus a demo surface split across two places: crates/multicalc/examples/ (headless, ships in the published tarball) and showcase/viz/ (the multicalc-viz live-Rerun crate). The layers themselves are each justified, but four problems make the setup confusing and expensive to scale.

What:

1. De-deplicate test configs using a new crate multicalc-testkit
2. embedded-smoke uses multicalc-testkit
3. Consolidate all tests into one binary
4. Create a single demos/ crate multicalc-demos. Move existing showcases and all examples into it.
5. Update all documentation.

Checklist

• CHANGELOG.md updated under ## [Unreleased] (required for behavior-facing changes)
• cargo test + cargo clippy --all-targets clean locally
• New public APIs have a doc example
• No unwrap/expect/panic on library paths (typed errors instead)

[kmolan]

@rtmongold for visibility. I streamlined all the tests and QA layers to be more scalable. You will need to rebase on top for the RISC-V changes. Let me know if there's any confusions, sorry for the code churn!