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/header.rs

@@ -0,0 +1,15 @@
+// SPDX-License-Identifier: GPL-3.0-or-later
+//! The Fury header parser never panics, and accepts only canonical headers:
+//! whatever it accepts re-serialises to exactly the bytes it consumed.
+#![no_main]
+use std::io::Read;
+use vault::crypto::header;
+
+libfuzzer_sys::fuzz_target!(|data: &[u8]| {
+    if let Ok((h, mut rest)) = header::parse(data) {
+        let mut tail = Vec::new();
+        rest.read_to_end(&mut tail).unwrap();
+        let consumed = &data[..data.len() - tail.len()];
+        assert_eq!(h.marshal(), consumed, "non-canonical header accepted");
+    }
+});