chore: cargo workspace skeleton

Eleven empty crates per DESIGN.md §11, versions and lints pinned once in the
root manifest, Justfile carrying the §12 gate, nix dev shell.

The frontend steps in `just ci` no-op until web/ exists. arr-core carries one
placeholder test because `cargo nextest` fails a zero-test run, and relaxing
that would let a broken filter go green later.

Closes #1

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

crates/arr-dl/Cargo.toml

@@ -0,0 +1,12 @@
+[package]
+name = "arr-dl"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+repository.workspace = true
+publish = false
+
+[dependencies]
+
+[lints]
+workspace = true

crates/arr-dl/src/lib.rs

@@ -0,0 +1 @@
+//! arr-dl — see DESIGN.md.