fix(substantive_fixer): WARN on corrupt eval_issues JSON

Third silent return path in substantive_fix_cycle — JSON-decode except
at the eval_issues parse drops rows that don't reach skipped_no_tags
or substantive_rows. If all 3 LIMIT-3 candidates have corrupt JSON,
cycle returns 0,0 with no log entry.

WARN level (not INFO): corrupt JSON is abnormal (post-merge column
drift, hand-edited DB row, partial write during crash). If this fires,
ops want to chase the upstream column-write path. If it never fires,
baseline noise stays at zero.

Closes the visibility gap on ALL silent returns in this function, not
just the two patched in 3f8666e.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

lib/substantive_fixer.py

@@ -544,6 +544,14 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
         try:
             issues = json.loads(row["eval_issues"] or "[]")
         except (json.JSONDecodeError, TypeError):
+            # Corrupt JSON in eval_issues is abnormal (post-merge column drift,
+            # hand-edited row, partial write during crash). WARN so ops can chase
+            # the upstream column-write path. Without this, the row drops out of
+            # both substantive_rows and skipped_no_tags — the third silent path.
+            logger.warning(
+                "PR #%d: corrupt eval_issues JSON — skipping in substantive fix cycle",
+                row["number"],
+            )
             continue
         if set(issues) & (FIXABLE_TAGS | CONVERTIBLE_TAGS | UNFIXABLE_TAGS):
             substantive_rows.append(row)