Cross builds, packaging split, CI, fuzzing, supply-chain policy

build.sh builds the Rust targets with the Go-era names, refuses an OUT
that is /, $HOME or the tree, checks static linking and names the Go
targets Rust cannot build. packaging/package.sh builds and inspects
packages without signing or publishing; repo.sh reuses it. Seven fuzz
targets (one found a URL-parser panic, fixed here with a regression
test), cargo-deny policy, CI for lint, tests on Linux/macOS/Windows,
cross builds with qemu runs, supply chain and fuzzing. Compatibility
specification and security notes.

Co-Authored-By: Claude <noreply@anthropic.com>

fuzz/fuzz_targets/tar.rs

@@ -0,0 +1,19 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+//! Walking arbitrary tar input as absorb does never panics, and every name
+//! it would store is confined.
+#![no_main]
+use std::io::Read;
+
+libfuzzer_sys::fuzz_target!(|data: &[u8]| {
+    let mut a = tar::Archive::new(data);
+    let Ok(entries) = a.entries() else { return };
+    for e in entries.take(64) {
+        let Ok(mut e) = e else { return };
+        let name = e.path_bytes().into_owned();
+        if let Some(n) = std::str::from_utf8(&name).ok().and_then(vault::absorb::safe_name) {
+            assert!(!n.split('/').any(|c| c == ".."));
+        }
+        let mut sink = Vec::new();
+        let _ = e.by_ref().take(1 << 20).read_to_end(&mut sink);
+    }
+});