fix(sec): A3 slice-2b review — no index cap, full discrepancy + malformed gate

1. Removed the 45-day index-walk cap: it discarded the older part of a long
   outage while still advancing source_max_date, permanently losing filings.
   The walk now covers every unprocessed date (a large gap is one-time cost).
2. Discrepancy detection meets the immutability contract: it compares ALL source
   snapshot fields (not five), read-only during stage/validate, reports the
   differing accessions + fields in validation_json, and promote emits a warning
   system event (in-transaction) — never mutating the stored row.
3. Malformed companyfacts (missing facts/units structure) are recorded separately
   and FAIL validation, instead of silently degrading to skipped rows that the
   50% backfill coverage floor could still pass.

Also corrected the stale "sum share classes" / DEI-only wording in the snapshot
model docstring and the A3 design doc to describe the us-gaap fallback.

Tests: +4 regressions (>45-day gap loses nothing, newly-added issuer backfills
without filing, malformed payload fails, shares discrepancy detected + evented).
23 passed, 1 skipped.

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

docs/dolt-sec-a3-design.md

@@ -101,8 +101,12 @@ One `fundamental_snapshots` row per accession, representing the filing's
   Q3≈9mo, FY≈12mo). **If the YTD fact is absent, store null — never a discrete
   masquerading as cumulative** (that would poison read-time differencing).
 - **Balance-sheet instants → at `end == reportDate`.** `shares_outstanding` is
-  the exception: take `dei:EntityCommonStockSharesOutstanding` for that accession
-  and store *its own* `end` in `shares_outstanding_date` (cover date ≠ period_end).
+  the exception: prefer the `dei:EntityCommonStockSharesOutstanding` cover-page
+  fact and store *its own* `end` in `shares_outstanding_date` (cover date ≠
+  period_end); when there is no dei fact (e.g. Alphabet) fall back to
+  `us-gaap:CommonStockSharesOutstanding` at `reportDate`. A single consolidated
+  value — never a class sum (companyfacts is non-dimensional) nor
+  weighted-average/diluted; conflicting values → null.
 - **Amendments:** a real `10-K/A` / `10-Q/A` is a new accession → a new immutable
   row for the same `(cik, fy, fp)`; readers pick the newest valid `accepted_at`.
 - **Out of scope (stated, not silent):** restatements that appear *only* as