fix: keep protobuf fields in release builds

[arpitagarwal1301]

Fixes #341

Summary

• preserve protobuf-lite generated fields needed by reflective schema parsing in minified release builds
• keep classes and methods eligible for R8 shrinking and obfuscation

Validation

• forced :app:minifyReleaseWithR8 :app:assembleRelease passed
• R8 mapping and final Dex retain bitField0_ and exerciseInfo_ while the message class remains obfuscated
• :app:testDebugUnitTest passed (48 tests, 14 skipped)
• git diff --check passed

Limitations

• Wear OS runtime validation was not available locally; only phone Android virtual devices are installed
• existing unrelated Spotless and lint errors remain

[gemini-code-assist]

Code Review

This pull request updates the ProGuard configuration file proguard-rules.pro by adding a rule to keep class members for any class extending com.google.protobuf.GeneratedMessageLite. There are no review comments, and I have no feedback to provide.

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