docs: update wasm target note and add common Cargo.lock

Reflect that wasm32-unknown-unknown is provided by the Nix dev shell
via rust-overlay rather than requiring a manual rustup install. Include
the Cargo.lock generated by building the common crate.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -54,7 +54,7 @@ vibesville/
 
 - **Framework:** Yew (Rust compiled to Wasm)
 - **Build tool:** Trunk (`trunk serve` for local dev)
-- **Compile target:** `wasm32-unknown-unknown` (must be installed: `rustup target add wasm32-unknown-unknown`)
+- **Compile target:** `wasm32-unknown-unknown` (included in the Nix dev shell via rust-overlay)
 - **Hosting:** Cloudflare Pages
 
 ### Data

common/Cargo.lock

@@ -0,0 +1,7 @@
+# This file is automatically @generated by Cargo.
+# It is not intended for manual editing.
+version = 4
+
+[[package]]
+name = "common"
+version = "0.1.0"