AGENT 367: Fix Mixed pub/non-pub Fields in Structs
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OBJECTIVE:
Fix 2 clippy warnings about mixed visibility in struct fields (partial_pub_fields)

ISSUES FOUND:
1. trading_engine/src/metrics.rs:315 - EnhancedHftLatencyTracker
   - Had: pub inner, pub metrics_buffer, private last_export_ns, private export_interval_ns
   - Fixed: All fields now private (inner and metrics_buffer were implementation details)

2. trading_engine/src/tracing.rs:257 - FastTracer  
   - Had: pub service_name, private span_queue, private dropped_spans, private spans_created, private spans_exported
   - Fixed: All fields now private (service_name accessed via methods)

CHANGES APPLIED:
1. trading_engine/src/metrics.rs:
   - Line 307: pub inner → inner (made private)
   - Line 309: pub metrics_buffer → metrics_buffer (made private)
   
2. trading_engine/src/tracing.rs:
   - Line 251: pub service_name → service_name (made private)

RATIONALE:
- Both structs expose public methods for accessing their internal state
- Making fields private enforces encapsulation and proper API boundaries
- No external code was directly accessing these fields (verified via grep)
- Internal code uses self.inner, self.metrics_buffer, self.service_name (still works)

VERIFICATION:
✅ cargo check passes
✅ No partial_pub_fields warnings remain
✅ All field access is through methods (encapsulation preserved)
✅ No external dependencies broken

FILES MODIFIED: 2
- trading_engine/src/metrics.rs (2 fields)
- trading_engine/src/tracing.rs (1 field)

RESULT: ✅ SUCCESS
- Mixed visibility warnings eliminated
- Proper encapsulation enforced
- No breaking changes to public API