feat(sec): A3 slice 2a — companyfacts -> snapshot parser

Pure parser (no I/O/DB) turning one issuer's companyfacts + submissions filing
metadata into per-accession snapshot rows for the filing's primary period.

- Period identity from end == reportDate, never fy/fp (fy/fp is the filing's
  context; comparatives repeat it).
- Duration facts stored as cumulative YTD: pick the fact whose span matches the
  fiscal-period-to-date length (Q1~3mo..FY~12mo) within tolerance; no YTD-length
  fact -> null (never a discrete masquerading as YTD).
- Balance-sheet instants at end == reportDate; shares_outstanding is the dei
  cover-page fact whose own end (cover date) is stored in shares_outstanding_date.
- Cash and debt composites are aggregate-first and mutually exclusive (each
  source tag counted at most once).
- Carries filing_date through submissions rows (snapshot.filed_date).

Verified on REAL Apple companyfacts: 44 snapshots, 0 skipped, YTD revenue
124.3B->219.7B->313.7B->416.2B across FY2025 (Q4 derives at read time), every
shares_date is the cover date != period_end. Tests: 6 fixture + 1 skip-guarded
live-invariants (monotonic YTD, cover-date shares).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

app/services/sec_client.py

@@ -270,6 +270,7 @@ def _rows_from_arrays(arrays: dict[str, list]) -> list[dict[str, Any]]:
                 "accession": arrays["accessionNumber"][i],
                 "form": form,
                 "report_date": arrays["reportDate"][i] or None,
+                "filing_date": arrays["filingDate"][i] or None,
                 "acceptance_datetime": arrays["acceptanceDateTime"][i] or None,
                 "is_xbrl": bool(arrays.get("isXBRL", [0] * len(forms))[i]),
             }