Fix DateTime add/subtract of weeks: 52 weeks is 364 days, not a calendar year

[c-schuler]

Problem

DateTime/Date arithmetic drifted by a day whenever a quantity of 52 or more weeks was added or subtracted. For example:

DateTime(2018, 5, 23) + 52 weeks // returned 2019-05-23, should be 2019-05-22

52 weeks is a fixed 364 days (and should preserve the day of week - Wed -> Wed), but the engine was returning a result 365 days later (the calendar anniversary).

Root cause

TemporalHelper.weeksToDays special-cased blocks of 52 weeks as 365-day calendar years:

if (weeks >= 52) { years = weeks / 52; weeks -= years * 52 }
return weeks * 7 + (years * 365)

This is correct for weeks < 52 (weeks * 7) but jumps by an extra day at every 52-week boundary (51 weeks → 357 days, 52 weeks → 365). A week is a fixed 7-day duration, so the year rollup has no basis in the spec.

Fix

fun weeksToDays(weeks: Int): Int = weeks * 7

Weeks are converted to days and added to the day component, letting the existing date logic walk the calendar (so leap-day/leap-year cases land correctly).

This matches the normative spec - CQL Author's Guide, Date/Time Arithmetic:

"The week, positive or negative, is multiplied by 7, and the resulting value to the day component, respecting calendar month and calendar year lengths."

(and the reference's "1 week = 7 days"), rather than any "52 weeks ~ 1 year" assumption.

Validation

• All six DateTime add/subtract-weeks conformance tests (cql-tests) now pass, including the leap-day and leap-year variants (DateTimeAddYearInWeeks, DateTimeLeapDayAddYear, DateTimeLeapYearAddYearInWeeks, and the Subtract counterparts).
• Full :engine:jvmTest suite passes with no regressions.

Scope

Fixes the add/subtract weeks path only. The "difference in weeks" issue (#1788 separate operator (DifferenceBetween) and is not addressed here.

Closes #1726, #1765

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Related Issues

The following open issues may be related to this PR:

Issue	Title	Score	Matched Terms
#1763	CqlComparisonOperatorsTest "groupName": "Unit Comparison" have many failures	21.5	"cql tests", "365 days", "week days", calendar, month, correct, duration, returning, year, 365, tests, special, days, week, engine (path), org (path), cql (path)
#1766	CqlDateTime Operators does not handle precision correctly	18	"cql tests", "time arithmetic", "date time", result, datetime, time, arithmetic, correctly, tests, date, example, org (path), cql (path)
#1026	Date/time addition for partials is incorrect in some cases	18	"365 days", "date time", cases, quantity, return, added, time, converted, year, spec, 365, date, days, value
#1767	Date/DateTime add/subtract behavior	17.5	"add subtract", "date time", "datetime add", quantity, month, add, datetime, time, subtract, date, value, cql (path)
#734	Width allows Interval as a parameter	16.5	"date time", result, operator, quantity, boundary, duration, datetime, time, difference, tests, date, value, example, engine (path), org (path), cql (path)

Tip: If this PR addresses any of these issues, please link them using Closes #NNN or Refs #NNN in the PR description.

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