feat: add DateUnit::Week and DateTimeUnit::Quarter to unit enums

[ZialeHub]

Summary

The current unit enums cover the most granular time units:

• DateUnit: Year, Month, Day
• DateTimeUnit: Year, Month, Day, Hour, Minute, Second

Two highly practical units are missing: Week and Quarter. These appear constantly in scheduling, reporting, and business logic, and consumers currently have to work around their absence by multiplying days or months manually — which is error-prone (e.g. "1 quarter" is not always 3 × 30 days).

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Proposed Addition

pub enum DateUnit {
    Year,
    Quarter,  // new
    Month,
    Week,     // new
    Day,
}

pub enum DateTimeUnit {
    Year,
    Quarter,  // new
    Month,
    Week,     // new
    Day,
    Hour,
    Minute,
    Second,
}

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Expected Behaviour

Week

A week is exactly 7 days. All existing Span methods must handle it:

// update
date.update(DateUnit::Week, 2)?;     // +14 days
date.update(DateUnit::Week, -1)?;    // -7 days

// next
date.next(DateUnit::Week)?;          // +7 days

// elapsed
let weeks = date_a.unit_elapsed(&date_b, DateUnit::Week)?;  // integer weeks between dates

// clear_unit
date.clear_unit(DateUnit::Week)?;    // rewind to Monday of the current week

Quarter

A quarter is 3 calendar months. It must respect month-end clamping (same as Month already does):

date.update(DateUnit::Quarter, 1)?;  // +3 months (2024-01-31 → 2024-04-30)
date.update(DateUnit::Quarter, -2)?; // -6 months

date.clear_unit(DateUnit::Quarter)?; // first day of the current quarter
// e.g. 2024-11-15 → 2024-10-01  (Q4 starts October)

let quarters = date_a.unit_elapsed(&date_b, DateTimeUnit::Quarter)?;

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Implementation Notes

• Week arithmetic delegates to DateUnit::Day with value * 7 — straightforward.
• Quarter arithmetic delegates to DateUnit::Month with value * 3.
• clear_unit(DateUnit::Week) should return the Monday of the week containing self (ISO week definition).
• clear_unit(DateUnit::Quarter) should return the first day of the quarter: Q1→Jan 1, Q2→Apr 1, Q3→Jul 1, Q4→Oct 1.
• unit_elapsed for Week = day_elapsed / 7 (integer division, truncated toward zero).
• unit_elapsed for Quarter = month_elapsed / 3.
• These additions are non-breaking — existing code using the current enums continues to compile.

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Acceptance Criteria

• DateUnit::Week and DateUnit::Quarter added
• DateTimeUnit::Week and DateTimeUnit::Quarter added
• update, next, clear_unit, unit_elapsed, and matches all handle the new variants
• clear_unit(Week) returns the ISO Monday of the current week
• clear_unit(Quarter) returns the first day of the current calendar quarter
• Edge cases tested: week crossing month/year boundary, Q4→Q1 rollover, leap year Q1
• Doc comments with examples on each new variant