chore: make the whole tree ruff-clean, not just app/

CI only lints app/, so 11 findings had accumulated in tests/ and scripts/.

Mechanical and behaviour-neutral, but two were not auto-fixable and needed a
judgement call rather than `ruff --fix`:

- E741 in run_fip_breadth_diagnostics: `l` is the OHLCV low and is genuinely
  used, so this was a naming fix (`l` -> `lo`), not a deletion.
- F841 in the same file: `vol_ix`/`momr_ix` are assigned from a pure local
  `_index()` and never read, so removing them cannot change any output. Their
  upstream `vol_weeks`/`momr_weeks` maps *are* used further down and stay; the
  comment above `_index` was corrected to say so.

The rest are unused imports and f-strings without placeholders (literal
markdown table headers, so identical output).

Verified beyond the linter, since py_compile does not catch a removed-but-used
import: every removed symbol has zero remaining references, all scripts compile,
and the full unit suite passes (852 passed, 1 skipped).

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

scripts/run_fip_breadth_research.py

@@ -162,8 +162,8 @@ def _write_md(path: Path, payload: dict) -> None:
     row = br.get("row") or br
     if row:
         lines.extend([
-            f"| metric | value |",
-            f"|---|---|",
+            "| metric | value |",
+            "|---|---|",
             f"| mean_ic | {row.get('mean_ic')} |",
             f"| ic_t_stat | {row.get('ic_t_stat')} |",
             f"| ic_positive_pct | {row.get('ic_positive_pct')} |",