fix(runtime): hold external sub-compositions in render mode

PR #917 fixed visibility clamping for external sub-compositions in
preview mode by checking data-composition-src. However, the producer's
htmlCompiler strips that attribute during inlining without setting the
data-composition-file marker that the core bundler sets. This caused
the runtime to still clamp duration to Math.min(authored, live) in
rendered output.

Two fixes:
- Runtime: also check data-composition-file (set by the core bundler
  after inlining)
- Producer: set data-composition-file before removing
  data-composition-src, matching the core bundler's behavior

Closes #911

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

packages/producer/src/services/htmlCompiler.ts

@@ -689,6 +689,7 @@ function inlineSubCompositions(
       host.innerHTML = contentDoc.toString();
     }
 
+    host.setAttribute("data-composition-file", srcPath);
     host.removeAttribute("data-composition-src");
 
     // Set explicit pixel dimensions on the host element so children using