feat: add Atlas tooling primitives

[leoafarias]

What changed

• add WidgetStateStyleOverride so tooling can render deterministic widget-state variants through normal Mix style resolution
• add Mix Protocol style/theme document inspection APIs
• preserve unresolved numeric token references when box constraints round-trip through the protocol
• keep the generated styler support barrel internal while exporting its public symbols through the normal Mix API

Why

This extracts the reusable Mix and Mix Protocol prerequisites from #982. The Atlas application and Atlas-owned packages live in btwld/muse#6, while producer validation remains in btwld/remix#68.

Closes the shared-core portion of #982 without merging Atlas product code into the Mix repository.

Validation

• fvm flutter test test/src/core/widget_state_style_override_test.dart in packages/mix
• fvm flutter test test/document_inspection_test.dart test/common_codecs_test.dart in packages/mix_protocol
• fvm flutter analyze packages/mix packages/mix_protocol