Placeholder values hiding as data: caller-declared sentinels and likely candidates

Overview

Scans columns of values for sentinels. Values you declare in a dictionary (for example -1 means unknown in column age) are reported as asserted hits with their row paths. Other values that look like missing-data markers are reported separately as candidates, never asserted: the pandas default NA strings, spreadsheet error values, whitespace-only text, placeholder words, zero and epoch dates, fixed-width integer limits, -1 in an otherwise non-negative column, runs of 9s above

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