De-obfuscate Dart stack traces from obfuscated builds

[ioannisj]

Extracted from PostHog/posthog#43012.

Problem

Frame chunk_id plumbing exists but the mobile provider always returns null, and there's no Dart symbol upload tooling, so --obfuscate / --split-debug-info builds produce unmapped frames. The Java/Kotlin path is already done.

Scope

• Upload Dart symbol files and map obfuscated frames back, both the build tooling and the backend resolution.