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>

app/models/fundamental_snapshot.py

@@ -20,11 +20,13 @@ class FundamentalSnapshot(Base):
     capex, depreciation_amortization) hold the filing's normalized **cumulative
     YTD/FY** value over (period_start -> period_end). Balance-sheet facts
     (cash_and_st_investments, total_debt, shares_outstanding) are **period-end**
-    values. ``shares_outstanding`` is a point-in-time count
-    (``dei:EntityCommonStockSharesOutstanding``, summed across share classes for
-    a multi-class issuer) — deliberately not the weighted-average diluted share
-    count, since both consumers (estimated market cap, YoY dilution read) want a
-    point-in-time value. Discrete quarters (10-Q YTD deltas, Q4 = FY - Q1..Q3), TTM, YoY and
+    values. ``shares_outstanding`` is a single consolidated point-in-time count —
+    the ``dei:EntityCommonStockSharesOutstanding`` cover-page fact, or
+    ``us-gaap:CommonStockSharesOutstanding`` at period end when no dei fact exists
+    (e.g. Alphabet). It is never a class sum (companyfacts is non-dimensional) nor
+    the weighted-average diluted count, since both consumers (estimated market cap,
+    YoY dilution read) want a point-in-time value. Discrete quarters (10-Q YTD
+    deltas, Q4 = FY - Q1..Q3), TTM, YoY and
     the quarter tape are all derived at read time — so non-calendar fiscal years
     resolve correctly and a later amendment never leaves a stale frozen quarter.
     """