Remove Kotlinx Datetime

[matt-ramotar]

kotlinx-datetime:0.7.0 removed Clock/Instant in favor of kotlin.time.Clock/Instant (requires Kotlin 2.1). Store still referenced kotlinx.datetime.Clock, so builds break when consumers pull in 0.7.0. See #703.

To move to kotlinx-datetime:0.7.0, we would need to move to Kotlin 2.1. The issue is that KMMBridge isn't aligned with Kotlin 2.1 (our other build plugins are, example CI failure).

Potential paths forward:

• (A) Replace kotlinx-datetime with our own method for epoch milliseconds. Store's public API only exposes epoch milliseconds. kotlinx.datetime.Instant is not in public types. This enables us to resolve the issue without bumping Kotlin or dropping KMMBridge.
• (B) Move to Kotlin 2.1, replace usages with kotlin.time.Clock, and align build plugins. This would require us to either drop KMMBridge, request a bump upstream, or temporarily maintain a fork of it.

This PR is implementing path A, which unblocks consumers immediately without requiring Kotlin 2.1, changes to KMMBridge, or any changes to our public API.

[itsandreramon]

when is this fix scheduled for release? is there a way to test nightly builds? @matt-ramotar

[matt-ramotar]

bugbot run

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.17%. Comparing base (d7af217) to head (bb5c087).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #706      +/-   ##
==========================================
+ Coverage   79.12%   79.17%   +0.04%     
==========================================
  Files          40       42       +2     
  Lines         939      941       +2     
  Branches      169      169              
==========================================
+ Hits          743      745       +2     
  Misses        120      120              
  Partials       76       76              

Files with missing lines	Coverage Δ
...tion/store/store5/impl/extensions/Clock.android.kt	100.00% <100.00%> (ø)
...nativefoundation/store/store5/StoreWriteRequest.kt	100.00% <100.00%> (ø)
...vefoundation/store/store5/impl/extensions/Clock.kt	100.00% <100.00%> (ø)
...undation/store/store5/impl/extensions/Clock.jvm.kt	100.00% <100.00%> (ø)

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[tyvsmith]

@gpolak did a big Clock convergence exercise internally, and may have some thoughts on long-term direction here.