fix(pipeline): MP1b - remove dead try/catch in PakWriter.Dispose

Self-review finding: Dispose()'s best-effort Finish() call was wrapped
in a try/catch for InvalidOperationException that could never fire —
the surrounding `if (!_finished)` guard already excludes the only
condition under which Finish() throws that exception. Dead defensive
code that LOOKS like it might be swallowing a real failure is exactly
the kind of thing the no-workarounds policy asks to avoid; removed with
a comment explaining why no catch is needed. Behavior unchanged; 43/43
Content.Tests still green.

src/AcDream.Content/Pak/PakWriter.cs

@@ -93,12 +93,12 @@ public sealed class PakWriter : IDisposable {
     }
 
     public void Dispose() {
+        // Best-effort finalize so a forgotten Finish() call doesn't leave a
+        // half-written file with a placeholder header claiming 0 entries
+        // while blobs are on disk. No try/catch needed: Finish() only throws
+        // when _finished is already true, which this guard already excludes.
         if (!_finished) {
-            // Best-effort finalize so a forgotten Finish() call doesn't leave a
+            Finish();
-            // half-written file with a placeholder header claiming 0 entries
-            // while blobs are on disk — surfaced loudly instead via Finish()'s
-            // own invariants on next use, but here we simply flush what exists.
-            try { Finish(); } catch (InvalidOperationException) { /* already finished */ }
         }
         _stream.Dispose();
     }