feat: Implement JSON Encoder

[zhexuany]

Issue Description

Currently, Robocodec supports reading JSON-encoded messages but does not support writing them. A JSON encoder is useful for:

• Debugging and inspection
• Web API integration
• Human-readable data export
• Cross-system compatibility

Proposed Solution

Implement a JsonEncoder to complement the existing JsonDecoder.

Requirements

• Add encoder.rs module in src/encoding/json/
• Implement MessageCodec trait for JsonEncoder
• Support encoding from CodecValue to JSON format
• Handle all primitive types and nested structures
• Proper array/sequence serialization
• Handle ROS time types (Time, Duration) correctly
• Add unit tests for round-trip encoding/decoding
• Add integration tests
• Update documentation

Acceptance Criteria

1. Can encode messages that were previously decoded by JsonDecoder
2. Round-trip tests pass: decode → encode → decode produces equivalent JSON
3. Output is valid, readable JSON
4. Handles complex nested types (sequences, nested messages, time types)

References

• Existing decoder: src/encoding/json/decoder.rs
• Codec interface: src/encoding/codec.rs
• CDR encoder reference: src/encoding/cdr/encoder.rs

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Priority: P1 - Core codec completeness
Estimated effort: Low-Medium