v0.4.1: Preserve recovered poll sources

This release preserves recovered poll-level source materials and historical coding evidence without changing existing knowledge or attitude estimates.

• Adds 37 registered files: 13 original historical scripts, additional Denmark/Europolis/UK–EU/BTP Health source versions and their dictionaries, original cross-poll inventory documents, and readable PDF companions.
• Retains Denmark's dated later follow-up as t3 in the wave catalog. Its knowledge answers are not scored until the changed question wording and answer keys are reviewed.
• Preserves three additional supplied BTP Health weight columns with original source identities, missing values, and zero weights. How to use weights remains undecided.
• Improves wide-workbook PDF previews so study names and headers remain readable across pages.

All 50 other generated output files are byte-identical to v0.4.0. Source-version differences and remaining instrument/linkage gaps are documented in docs/poll-issues.md; this release does not claim that all poll corrections are finished.

Validation: local metadata validation, full R/Python tests, and lint passed; an independent review verified 37 source checksums and original archive copies, reproduced the weight and wave tables, and passed 1,060 focused assertions. GitHub checks passed for both the reviewed candidate and merged commit e92b9aa.