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>

reports/fundamentals-parity-20260723-findings.md

@@ -801,6 +801,29 @@ data limitation and two operational steps that only run against production.
 8. ~~**PPL basic-EPS fallback**~~ — **done**, ninth pass.
 9. ~~**HAL null TTM EPS**~~ — **resolved**, ninth pass (already fixed; 2024 is a filer error).
 
+## Review finding — two fixes on this branch silently interacted
+
+Caught in review, not by me. `_merge_amendments` (the per-field amendment fix, first pass)
+builds the merged period from `_MERGED_FIELDS` + `_CARRIED_FIELDS` alone, so a column in
+neither list is **absent** from the merged row, not merely stale — and every caller reads it
+with `getattr(row, name, None)`, which quietly returns `None`.
+
+`weighted_avg_diluted_shares` (the market-cap fallback, seventh pass) was never added to
+`_MERGED_FIELDS`. The failure needed both fixes to be present at once: a multi-class issuer
+*and* 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, i.e. the seventh pass's fix undone by the
+first pass's mechanism. I updated `_SNAPSHOT_COLS` in the importer when adding the column
+but not `_MERGED_FIELDS` in the derivation.
+
+Fixed, with a regression test for the specific case. The more useful addition is a guard —
+`test_merge_lists_cover_every_parser_field` asserts the two lists cover every `SnapshotRow`
+field, so the *next* column added fails loudly instead of losing data quietly. Verified it
+would have caught this one.
+
+Lesson worth keeping: a hand-maintained field list that reconstructs an object is a silent
+data-loss footgun. `_SNAPSHOT_COLS` (importer) and `_MERGED_FIELDS` (derivation) must both
+track the parser's `SnapshotRow`, and only one of them is now enforced by a test.
+
 ## Genuinely unfixable from this data
 
 - **§1 part (b)** — a split post-dating the last filing (KLAC). No snapshot carries