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]>

build.rs

@@ -0,0 +1,32 @@
+//! Build script: records the git commit cumulus is built from, so
+//! `cumulus --version` can tell exactly which build you're running.
+//!
+//! Outside a git checkout, or without git installed, the commit is "unknown".
+//! (No Windows icon or manifest: a terminal program's icon only shows in
+//! Explorer, and a manifest changes nothing for it.)
+
+use std::path::Path;
+use std::process::Command;
+
+fn main() {
+    let commit = git_commit().unwrap_or_else(|| "unknown".to_string());
+    println!("cargo::rustc-env=CUMULUS_COMMIT={commit}");
+
+    // Run again when the commit changes: HEAD moves on checkout, and the
+    // branch files change on commit. Only watch files that exist, or cargo
+    // would run this script on every build.
+    println!("cargo::rerun-if-changed=build.rs");
+    for path in [".git/HEAD", ".git/refs/heads", ".git/packed-refs"] {
+        if Path::new(path).exists() {
+            println!("cargo::rerun-if-changed={path}");
+        }
+    }
+}
+
+/// The short hash of the current commit, if git can tell.
+fn git_commit() -> Option<String> {
+    let output = Command::new("git").args(["rev-parse", "--short=9", "HEAD"]).output().ok()?;
+    let hash = String::from_utf8(output.stdout).ok()?;
+    let hash = hash.trim();
+    (output.status.success() && !hash.is_empty()).then(|| hash.to_string())
+}