Add with helper for immutable rule updates

[ggaabe]

Summary

• add a with() helper to create modified RRuleTemporal instances without mutating originals
• clone existing options arrays before merging overrides to avoid shared references
• cover the new behavior with unit tests

Testing

• npm test

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Codex Task

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Note

Introduces immutable rule updates and strengthens RSCALE handling.

• Adds with(updates) to RRuleTemporal, merging updates over cloned option arrays via cloneOptions/cloneUpdateOptions to avoid shared references
• Caches and enforces non-Gregorian RSCALE calendar support with rscaleCalendarSupport and assertRscaleCalendarSupported, invoked before RSCALE engines
• New tests: rfc7529_rscale_non_gregorian.test.ts (with calendar support probing) and rrule-temporal.test.ts cases validating with() immutability and rDate/exDate cloning

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[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Clone update arrays to keep with() immutable

The new with() helper spreads updates directly into merged, so array fields provided by the caller are stored by reference. This is observable for fields that the constructor does not sanitize/clone (notably rDate and exDate): if a caller does const r = rule.with({rDate}); rDate.push(...), the updated rule’s options mutate after construction, which violates the stated immutability goal and can change subsequent recurrence results. Consider cloning array-valued updates (or specifically rDate/exDate) before constructing the new instance.

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