refactor(checkpoints): Add DeviceDimensionProvider (#3943)

### Description

Adds the first dimension provider, so rule predicates have something to
read: `device.appVersion`, `device.locale`, `device.platform`,
`device.platformVersion` and `device.sdkVersion`.



<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> **Medium Risk**
> Enables real checkpoint audience matching on device/locale/platform
fields; camelCase dimension keys are a new predicate contract, though
behavior is additive with tests covering provider and platform mapping.
> 
> **Overview**
> **Local rules can now read device context.** `PurchasesFactory` wires
`DeviceDimensionProvider` into `LocalRulesEvaluator` (replacing an empty
provider list), so checkpoint predicates can target `device.appVersion`,
`device.locale`, `device.platform`, `device.platformVersion`, and
`device.sdkVersion`.
> 
> `DeviceDimensionProvider` snapshots most values at configure time but
**re-reads locale** on each evaluation (normalized to lowercase
`en_us`), omits empty string dimensions, and maps Amazon vs other stores
via shared `Store.platformName`. That same extension replaces
`HTTPClient`'s private `X-Platform` mapping so headers and rules stay
aligned.
> 
> Dimension key documentation and resolver tests move from **snake_case
to camelCase** (`appVersion` vs `app_version`) to match the nested
`device.*` paths the engine resolves.
> 
> <sup>Reviewed by [Cursor Bugbot](https://cursor.com/bugbot) for commit
d52917764ec5bf1b8d1fc0eb864fb189777fff62. Bugbot is set up for automated
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<!-- /CURSOR_SUMMARY -->

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>