Preserve \after:\ field when executor rewrites .dvc files

The executor's \`write_dvc_file\` calls were not passing
\`artifact.computation.after\`, silently dropping ordering constraints
when .dvc files were rewritten after execution.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: ryan-williams

specs/done/preserve-after-on-rewrite.md

@@ -0,0 +1,46 @@
+# Bug: `after:` field dropped when DVX rewrites `.dvc` files
+
+## Problem
+
+When DVX runs a stage and rewrites its `.dvc` file (to update dep hashes, `last_run`, etc.), the `after:` ordering field is silently dropped.
+
+## Root cause
+
+`write_dvc_file` in `src/dvx/run/dvc_files.py:484` accepts an `after` parameter, but the call sites in the executor (`src/dvx/run/executor.py:476` and `:530`) don't pass it:
+
+```python
+dvc_file = write_dvc_file(
+    output_path=Path(path),
+    cmd=cmd if self.config.provenance else None,
+    deps=deps_hashes if self.config.provenance else None,
+    git_deps=git_deps_hashes if self.config.provenance else None,
+    fetch_schedule=fetch_schedule,
+    fetch_last_run=fetch_last_run,
+    # after= is missing!
+)
+```
+
+The `after` info is available on `artifact.computation.after` (populated from the original `.dvc` file), but never passed through.
+
+## Fix
+
+At both `write_dvc_file` call sites in `executor.py`, pass `after=artifact.computation.after`.
+
+## Repro
+
+Any `.dvc` file with an `after:` block will lose it after `dvx run`:
+
+```yaml
+# before
+meta:
+  computation:
+    cmd: some_command
+    after:
+      - other/stage.dvc
+
+# after dvx run
+meta:
+  computation:
+    cmd: some_command
+    # after: gone
+```

src/dvx/run/executor.py

@@ -463,6 +463,9 @@ def _execute_artifact(self, artifact: Artifact) -> ExecutionResult:
             from datetime import datetime, timezone
             fetch_last_run = datetime.now(timezone.utc).isoformat()
 
+        # Preserve after: ordering constraints across rewrites
+        after = artifact.computation.after if artifact.computation else []
+
         if is_side_effect:
             # Side-effect: update dep hashes in .dvc, no output hash
             dvc_file = None
@@ -480,6 +483,7 @@ def _execute_artifact(self, artifact: Artifact) -> ExecutionResult:
                     git_deps=git_deps_hashes if self.config.provenance else None,
                     fetch_schedule=fetch_schedule,
                     fetch_last_run=fetch_last_run,
+                    after=after or None,
                 )
                 if self.config.verbose:
                     self._log(f"       → {dvc_file}")
@@ -536,6 +540,7 @@ def _execute_artifact(self, artifact: Artifact) -> ExecutionResult:
                 git_deps=git_deps_hashes if self.config.provenance else None,
                 fetch_schedule=fetch_schedule,
                 fetch_last_run=fetch_last_run,
+                after=after or None,
             )
             if self.config.verbose:
                 self._log(f"       → {dvc_file}")
@@ -587,13 +592,15 @@ def _handle_co_output(self, artifact: Artifact, cmd: str) -> ExecutionResult:
                 deps_hashes = artifact.computation.get_dep_hashes()
                 git_deps_hashes = artifact.computation.get_git_dep_hashes()
 
+            co_after = artifact.computation.after if artifact.computation else []
             dvc_file = write_dvc_file(
                 output_path=out,
                 md5=md5,
                 size=size,
                 cmd=cmd if self.config.provenance else None,
                 deps=deps_hashes if self.config.provenance else None,
                 git_deps=git_deps_hashes if self.config.provenance else None,
+                after=co_after or None,
             )
 
             self._log(f"  ✓ {path}: co-output ready")