Port alknet-typedef crate from alknet

Copy the binary struct engine (src/, tests/) verbatim from
alknet/crates/alknet-typedef and create a standalone Cargo.toml
(workspace-inherited fields inlined). Port the architecture docs
(specs, ADRs 095-102, OQs 069-071) from alknet's nested multi-crate
layout to a flat single-crate layout, fixing relative link paths.

Build, 295 tests, and clippy all pass clean.

.gitignore

@@ -0,0 +1,3 @@
+target/
+node_modules/
+.worktrees/