feat: add start_of / end_of period convenience methods

[ZialeHub]

Summary

clear_unit currently resets a field to its minimum value (day → 1, month → 1, hour → 0). This covers start_of_* semantics. However, the equally important end_of_* operations are missing entirely, and consumers are forced to reimplement non-trivial calendar logic — particularly end_of_month, which must handle varying month lengths and leap years correctly.

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Proposed API

On Date

date.start_of_month()? -> Date   // 2024-10-15 → 2024-10-01
date.end_of_month()?   -> Date   // 2024-10-15 → 2024-10-31
                                 // 2024-02-10 → 2024-02-29  (leap year)
                                 // 2023-02-10 → 2023-02-28

date.start_of_year()?  -> Date   // 2024-10-15 → 2024-01-01
date.end_of_year()?    -> Date   // 2024-10-15 → 2024-12-31

date.start_of_week()?  -> Date   // ISO: Monday of current week
date.end_of_week()?    -> Date   // ISO: Sunday of current week

date.start_of_quarter()? -> Date // 2024-11-15 → 2024-10-01  (Q4)
date.end_of_quarter()?   -> Date // 2024-11-15 → 2024-12-31  (Q4)

On DateTime

Same methods, preserving or zeroing time components as appropriate:

datetime.start_of_month()? -> DateTime  // sets day=1, time=00:00:00
datetime.end_of_month()?   -> DateTime  // sets day=last, time=23:59:59
datetime.start_of_day()?   -> DateTime  // sets time=00:00:00  (alias for clear_time)
datetime.end_of_day()?     -> DateTime  // sets time=23:59:59

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Why Not Just Use clear_unit?

clear_unit is a low-level building block. These methods are higher-level semantic operations:

Goal	With clear_unit	With new methods
First day of month	date.clear_unit(DateUnit::Day)?	date.start_of_month()?
Last day of month	Manual: compute days in month, then set	date.end_of_month()?
First day of quarter	Manual: compute quarter start month, then set	date.start_of_quarter()?
Last second of day	Manual: update to 23:59:59	datetime.end_of_day()?

end_of_month in particular is non-trivial: it requires knowing the number of days in the month, which depends on the year (leap year handling for February). This is exactly the kind of logic timeflow should own.

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Implementation Notes

• end_of_month should use chrono's NaiveDate::from_ymd_opt with the next month's first day minus one day — the idiomatic approach that handles all edge cases automatically.
• start_of_week / end_of_week should follow ISO 8601 (Monday = start, Sunday = end).
• Quarter boundaries: Q1 = Jan–Mar, Q2 = Apr–Jun, Q3 = Jul–Sep, Q4 = Oct–Dec.
• All methods return Result<Self, SpanError> for consistency with the existing API.
• These methods can be implemented in terms of existing primitives (clear_unit, update, new) to minimise new code surface.

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Acceptance Criteria

• start_of_month, end_of_month, start_of_year, end_of_year implemented on Date and DateTime
• start_of_week, end_of_week implemented (ISO Monday–Sunday)
• start_of_quarter, end_of_quarter implemented
• start_of_day, end_of_day implemented on DateTime
• end_of_month correctly handles February in leap and non-leap years
• end_of_month correctly handles all 28/29/30/31-day months
• Each method has a doc comment with at least one rust,ignore example
• Full test coverage including leap year February, Q4 end, week crossing year boundary