fix(521): de-dup 6 overlapped records; guard duplicate metadata blocks; exclude retrieval-eval; complete eval bank [decisions-edit]

- Exclude docs/decisions/retrieval-eval.md from active decision parsing
  (_NON_DECISION_FILES); its `## N.` eval-question headings were being
  miscounted as 7 legacy-unmigrated records.
- Add decisions_lib.metadata_line_count() + a decisions_validate guard
  that fails a record with more than one `key:` metadata line, so a
  stacked-metadata-block migration bug (which the parser silently
  tolerated by reading only the first block) can't recur unnoticed.
  TDD: test_duplicate_metadata_block_fails / test_single_metadata_block_passes.
- De-duplicate the 6 docs/decisions.md records left with two stacked
  metadata blocks (scan.getoraddfolder-db-lookup #488,
  scan.musicvideo-reconciliation #494, scan.jellyfin-mixed-content-library
  #489, iptv.logo-drives-bug-preset #67, ffmpeg.qsv-decode-encode-split
  #498, ci.small-lane-git-only server-management#639), merging the union
  of Signals/paths/issues/Mechanics from both blocks and keeping the
  richer Rule wording; rationale prose untouched.
- Fill in the deferred Q6b row in docs/decisions/retrieval-eval.md now
  that startup.parallel-orientation is active in docs/decisions.md,
  scoring it as a real active-vs-superseded question against the
  archived docs.queue-state-gitea-tracker.
- Regenerate docs/decisions/README.md via build_decisions_catalog.py.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

scripts/decisions_lib.py

@@ -98,7 +98,18 @@ def parse_file(path: Path) -> list[Record]:
     return parse_text(path.read_text(encoding="utf-8"), path)
 
 
-_NON_DECISION_FILES = {"README.md", "migration-map.md"}
+def metadata_line_count(rec: Record) -> int:
+    """Count how many lines in `rec.body` look like a metadata line (a line starting with
+    `` `key: `` after stripping). A well-formed record has exactly 1; more indicates a
+    duplicate/stacked metadata block left behind by a botched migration."""
+    count = 0
+    for line in rec.body.splitlines():
+        if line.strip().startswith("`key:"):
+            count += 1
+    return count
+
+
+_NON_DECISION_FILES = {"README.md", "migration-map.md", "retrieval-eval.md"}
 
 
 def active_files() -> list[Path]: