Allow @def body to start on the next continuation line

Previously, a backslash immediately after the macro name (with no
body on the first line) produced a leading newline. Now an empty
prefix is handled so the body begins cleanly on the continuation line.

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

README.md

@@ -110,7 +110,7 @@ $ greet
 hello world
 ```
 
-Use backslash `\` for multi-line bodies:
+Use backslash `\` for multi-line bodies. The body can start on the same line or on the next continuation line:
 
 ```
 @def serve \

src/parse.test.ts

@@ -204,6 +204,14 @@ describe("parse", () => {
     expect(result.commands).toHaveLength(1)
   })
 
+  test("@def with body starting on continuation line", () => {
+    const content = "@def serve \\\n  python3 -m http.server\n$ serve\n"
+    const result = parse("test.shout", content)
+    expect(result.directives).toEqual([
+      { type: "def", name: "serve", body: "python3 -m http.server", line: 1 },
+    ])
+  })
+
   test("@def multiple macros", () => {
     const content = "@def foo echo foo\n@def bar echo bar\n$ foo\nfoo\n"
     const result = parse("test.shout", content)

src/parse.ts

@@ -91,7 +91,8 @@ function parseDefDirective(
     if (next.startsWith("$") || next.startsWith("@") || next.trim() === "" || next.startsWith("#")) {
       throw new Error(`${path}:${i + extra + 2}: @def continuation consumed a command or directive line`)
     }
-    body = body.slice(0, -1).trimEnd() + "\n" + next.trim()
+    const prefix = body.slice(0, -1).trimEnd()
+    body = prefix ? prefix + "\n" + next.trim() : next.trim()
     extra++
   }
 

web/index.html

@@ -225,7 +225,7 @@
 
 <span class="prompt">$</span> <span class="cmd">greet</span>
 <span class="output">hello world</span></code></pre>
-  <p>If a command matches a macro name exactly, the body is substituted. Use <code>\</code> for multi-line bodies. Macros from setup files are inherited; user-file macros override them.</p>
+  <p>If a command matches a macro name exactly, the body is substituted. Use <code>\</code> for multi-line bodies — the body can start on the same line or on the next continuation line. Macros from setup files are inherited; user-file macros override them.</p>
 </section>
 
 <section>