Module Aggregator/Metadata/DateLabels

Description

Pure date-label derivation helpers extracted from article-metadata.ts as a leaf module in the metadata/ bounded context. Every helper takes an ISO YYYY-MM-DD string and returns a human-friendly label (or {start, end} window) used by the per-article-type template-fallback title generators.

Bounded-context rules for this file:

• No upward imports — pure helpers, no dependencies on other src/aggregator/ modules, no I/O, no globals.
• Deterministic — same input always produces same output; safe to call from property-based tests.
• UTC-only — all parsing/formatting goes through Date UTC accessors, never local-time getMonth()/getDate().

Cross-references:

• EP-term boundary constants follow analysis/methodologies/electoral-cycle-methodology.md.
• The D-36 → D-8 reporting window for week-in-review follows ADR-006 (EP roll-call publication lag).