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>

.gitignore

@@ -7,3 +7,7 @@
 /docs/guide/vault-guide.pdf
 /docs/guide/*.log
 /target
+fuzz/target
+fuzz/corpus
+fuzz/artifacts
+fuzz/coverage