refactor(core): move global environment behind KlaviyoEnv holder for Swift 6

[ndurell]

Description

Step 1.1 of the Swift 6 strict-concurrency migration. Replaces the public mutable global environment at Sources/KlaviyoCore/KlaviyoEnvironment.swift:13 with a typed holder KlaviyoEnv.current, gated behind @_spi(KlaviyoPrivate). The original environment symbol remains as a deprecated SPI shim for source compatibility while call sites migrate (chunk 1.2.*).

Due Diligence

• I have tested this on a simulator or a physical device.
• I have added sufficient unit/integration tests of my changes.
• I have adjusted or added new test cases to team test docs, if applicable.
• I am confident these changes are compatible with all iOS and XCode versions the SDK currently supports.

(No new tests — this is a pure refactor with zero behavior change. Existing test suite passes.)

Release/Versioning Considerations

• Patch Contains internal changes or backwards-compatible bug fixes.
• Minor Contains changes to the public API.
• Major Contains breaking changes.
• Contains readme or migration guide changes.
• This is planned work for an upcoming release.

The new KlaviyoEnv symbol and the deprecated environment shim are both @_spi(KlaviyoPrivate) — they can only be reached via @_spi(KlaviyoPrivate) import KlaviyoCore. The pre-existing environment global was plain public; tightening it to SPI is a deliberate access narrowing. All call sites in this repo (and our sibling test apps, which already use the SPI import) are unaffected.

Changelog / Code Overview

New canonical entrypoint:

@_spi(KlaviyoPrivate) public enum KlaviyoEnv {
    nonisolated(unsafe) public static var current = KlaviyoEnvironment.production
}

• nonisolated(unsafe) is the documented Apple stopgap for nonisolated global shared mutable state under -strict-concurrency=complete. Race surface is unchanged — labeled, not eliminated. Real isolation lands when the closures inside KlaviyoEnvironment get rehomed into per-feature containers in later chunks.

Deprecated source-compat shim:

@_spi(KlaviyoPrivate)
@available(*, deprecated, renamed: "KlaviyoEnv.current")
public var environment: KlaviyoEnvironment {
    get { KlaviyoEnv.current }
    set { KlaviyoEnv.current = newValue }
}

Knock-on changes required by the SPI gating:

• All cross-module imports of KlaviyoCore flipped to @_spi(KlaviyoPrivate) import KlaviyoCore (40 source + test files). Mechanical change; no call-site edits.

• Three public inits in KlaviyoCore reference environment in their default-argument values, which the compiler rejects for non-SPI public ABI (default args become part of the public interface):

  • KlaviyoAPI.init(send:)
  • KlaviyoRequest.init(id:endpoint:)
  • AppLifeCycleEvents.init(lifeCycleEvents:)

  These are gated @_spi(KlaviyoPrivate) to match. All current callers already have SPI access via the import flip; customer-facing surface (KlaviyoSDK, Klaviyo.shared, etc.) is unchanged.

• Dropped @usableFromInline on runtimeWarn in LoggerClient.swift — no @inlinable callers exist in KlaviyoCore, and the annotation was inadvertently exposing its default args to the cross-module surface.

What chunk 1.2. will do (out of scope here):*

• Migrate ~30 in-repo environment.X call sites to KlaviyoEnv.current.X (deprecation warnings flag them today).
• Delete the deprecated environment shim once call sites are migrated.

Out of scope for the broader 1.x effort, separate chunks:

• The networkSession global at KlaviyoEnvironment.swift:300.
• Sibling test apps in klaviyo-{ios,swift}-test-app (no changes needed — they already use @_spi(KlaviyoPrivate) import KlaviyoCore).

Test Plan

• xcodebuild build -scheme klaviyo-swift-sdk-Package -destination "platform=iOS Simulator,name=iPhone 17 Pro" — succeeds, with the expected deprecation warnings on existing environment.X call sites (to be cleared in chunk 1.2.*).
• make test-library — full test suite passes.
• Verified deprecation warnings flow through ('environment' is deprecated: renamed to 'KlaviyoEnv.current') on all in-repo call sites.

Related Issues/Tickets

Swift 6 strict concurrency initiative.