docs(xtask): point the layers help text at docs/architecture.md

cargo xtask --help and the layers command output both referred to plan/architecture.md, which does not exist in this repository; there is no plan/ directory at the root. The dependency layering rules are documented as section 3 of docs/architecture.md, so point both references there.

xtask/src/main.rs

@@ -22,7 +22,7 @@ const USAGE: &str = "\
 usage: cargo xtask <command>
 
 commands:
-  layers          enforce the crate dependency layering (plan/architecture.md §3)
+  layers          enforce the crate dependency layering (docs/architecture.md §3)
   wasm            cargo check --target wasm32-unknown-unknown for the wasm-safe crates
   ci              fmt --check, clippy -D warnings, test, layers, wasm (stops at first failure)
   corpus [--all | --pngsuite | --psd | --psd-tools | --pixls | --photoshop] [--local] [--update-manifest]
@@ -106,7 +106,7 @@ pub fn metadata() -> Result<serde_json::Value, String> {
 
 fn cmd_layers() -> Result<(), String> {
     let crates = layers::from_metadata(&metadata()?)?;
-    println!("Dependency layering (plan/architecture.md §3)\n");
+    println!("Dependency layering (docs/architecture.md §3)\n");
     println!("{:<28} {:<14} workspace deps", "crate", "layer");
     for c in &crates {
         let ws: Vec<String> = c