fix(sec): keep the market-cap fallback through an amendment merge

Found in review. _merge_amendments rebuilds a period from _MERGED_FIELDS +
_CARRIED_FIELDS alone, so a column in neither list is absent from the merged
row, not just stale — and callers read it with getattr(..., None), which
silently yields None. weighted_avg_diluted_shares was never added when the
market-cap fallback landed (_SNAPSHOT_COLS in the importer was updated, its
counterpart in the derivation was not).

The failure needed both of this branch's fixes at once: a multi-class issuer
with a partial amendment on its latest period (META with a Part-III-only
10-K/A) would silently lose market cap and FCF yield again.

Adds the field, a regression test for that case, and a guard test asserting
the merge/carry lists cover every SnapshotRow field, so the next column added
fails loudly rather than losing data quietly. Confirmed the guard catches the
original bug.

Also from review:
- Expose pe_caveat in the valuation payload, so a P/E suppressed by split
  contamination says why instead of looking like missing data (the caveat was
  set but never read).
- no_xbrl_filings now names both causes; the old text advised pinning a CIK
  override, which is wrong for a genuine new registrant that simply has not
  filed yet and clears itself.
- Document that fiscalYearEnd is the issuer's current calendar, so a fiscal-
  year-end change degrades old periods (fallback/newest-wins), not current ones.
- Parser-level tests for _select_weighted_avg_shares (shortest-span-wins and
  concept priority), which only had derivation-level coverage.

823 unit tests pass.

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

app/services/sec_facts_parser.py

@@ -310,6 +310,12 @@ def _period_identity(
     ends in early January (DPZ) shifts by one — they need to be unique, monotonic
     and YoY-aligned, which is all the derivation asks of them. Nothing outside the
     derivation reads these columns.
+
+    Known limitation: ``fiscalYearEnd`` is the issuer's *current* calendar, so a
+    company that has changed its fiscal year end gets its historical periods
+    measured against the new one. The quarter tolerance shunts most of those to
+    the fy/fp fallback, and a same-key collision resolves newest-wins, so the
+    failure mode is a degraded old year rather than a scrambled current one.
     """
     fy = _fiscal_year_of(meta.report_date, fiscal_year_end)
     if fy is None: