Refactored conditional layer composition to use Option<Layer> pattern:
- Create console_layer and file_layer as Option<Layer> types
- Build subscriber with .with(console_layer).with(file_layer)
- Eliminates type mismatch from conditional registry.with() calls
This fixes the E0308 error at line 194 where the compiler expected
struct Layer but found enum Option. The tracing-subscriber crate
properly handles Option<Layer> in .with() calls, making conditional
layer composition type-safe.
Verified:
- cargo check -p common: passes
- cargo test -p common --lib: 158/158 tests passing
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