Add the project skeleton and the approved design

Cargo.toml with the release profile and lints, the toolchain pinned to
Rust 1.90.0, a build script that records the git commit for
`cumulus --version`, and rustfmt settings and cargo aliases (`cargo lint`,
`cargo linux`) as in skyshift. DESIGN.md holds the design approved after
the question rounds; README.md has build instructions and the planned keys.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

rustfmt.toml

@@ -0,0 +1,4 @@
+# `cargo fmt` formats all the code the same way. Run it after editing.
+max_width = 110
+# Keep short expressions on one line (like `if a { b } else { c }`).
+use_small_heuristics = "Max"