music-history-db: Redesign date storage for uncertainty and multiple dates

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Finding

Partitioning events only under data/events/MM/DD.jsonl does not naturally represent unknown month/day, date ranges, disputed dates, multiple territory-specific release dates, or the distinction between an occurrence date and an anniversary/display date.

Relevant sections: proposed repository structure, event fields, date tests, and publication gates in tools/specs/music-history-db/KSER_Music_History_Technical_Implementation_Strategy.md.

Required decision

Define canonical date semantics and storage that preserve uncertainty and conflicting source assertions without forcing an unsupported calendar day.

Acceptance criteria

• Distinguish occurrence dates, display/anniversary dates, and source-claimed dates.
• Represent precision, uncertainty, ranges, circa values, and unknown components.
• Represent multiple territory-specific or otherwise qualified dates.
• Preserve conflicting assertions without selecting a winner before review.
• Choose a source-controlled partitioning scheme for records without a known month/day.
• Add tests for February 29, partial dates, ranges, conflicts, and territory-specific dates.