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

@@ -539,15 +539,36 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
 
     # Filter to only PRs with substantive issues (not just mechanical)
     substantive_rows = []
+    skipped_no_tags = []
     for row in rows:
         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)
+        else:
+            skipped_no_tags.append((row["number"], issues))
 
     if not substantive_rows:
+        # Visibility for the LIMIT-3 head-of-line block: if the oldest
+        # candidates have no fixer-actionable tags (e.g. eval_issues=[],
+        # broken_wiki_links only), the cycle silently returns 0 — and the
+        # next cycle picks the same head-of-line, forever. Log the eval_issues
+        # of skipped candidates so the journal makes the block visible.
+        if skipped_no_tags:
+            logger.info(
+                "Substantive fix cycle: 0 actionable from %d candidate(s) — head-of-line: %s",
+                len(rows), skipped_no_tags,
+            )
         return 0, 0
 
     fixed = 0
@@ -559,7 +580,13 @@ async def substantive_fix_cycle(conn, max_workers=None) -> tuple[int, int]:
             if result.get("action"):
                 fixed += 1
             elif result.get("skipped"):
-                logger.debug("PR #%d: substantive fix skipped: %s", row["number"], result.get("reason"))
+                # Was DEBUG — promoted to INFO to make stuck-PR root cause
+                # visible without enabling DEBUG fleet-wide. (Ship Apr 24+
+                # silent skip diagnosis.)
+                logger.info(
+                    "PR #%d: substantive fix skipped: %s",
+                    row["number"], result.get("reason"),
+                )
         except Exception:
             logger.exception("PR #%d: substantive fix failed", row["number"])
             errors += 1