Initial nix-ota implementation

Self-hostable OTA update system for NixOS fleets: a control server,
device agent, publisher CLI, and NixOS modules that ship prebuilt
system closures from a binary cache to devices that don't have the
flake.

- crates/common: signed manifest types (ed25519), store-path validator
- crates/server: axum + sqlite + HTMX dashboard, channel/device API
- crates/agent: poll, verify signature + revision, nix copy, switch,
  health check, magic-rollback on failure
- crates/publisher: keygen + sign + publish CLI for operators/CI
- nix/modules: NixOS modules for server and agent
- nix/tests/ota.nix: end-to-end VM test exercising publish A -> B ->
  broken C -> rollback to B (passes)

The control server never holds the signing key; manifests are signed
offline and verified against a pinned public key on each device.

crates/common/Cargo.toml

@@ -0,0 +1,18 @@
+[package]
+name = "nix-ota-common"
+version.workspace = true
+edition.workspace = true
+license.workspace = true
+
+[lib]
+path = "src/lib.rs"
+
+[dependencies]
+serde.workspace = true
+serde_json.workspace = true
+ed25519-dalek.workspace = true
+base64.workspace = true
+rand.workspace = true
+time.workspace = true
+thiserror.workspace = true
+sha2.workspace = true