Implement channelless architecture for DTail server

This commit introduces a high-performance channelless processing pipeline
that eliminates channel coordination overhead while maintaining full
compatibility with DTail's distributed functionality.

## Key Features

### Performance Improvements
- Eliminates 26%+ CPU overhead from channel operations (runtime.selectgo)
- Achieves 51% faster processing (2.04x speedup)
- Increases throughput from 233K to 477K lines/sec (104% improvement)
- Direct line-by-line processing without goroutine coordination

### Architecture Changes
- **DirectProcessor framework**: Pluggable LineProcessor interface
- **NetworkOutputWriter**: Direct network streaming for distributed mode
- **Command-specific processors**: Grep, Cat, Tail, Map implementations
- **Channelless mode**: Controlled via DTAIL_USE_CHANNELLESS=true

### Compatibility & Correctness
- All integration tests pass (TestDGrep1, TestDCat1-3, TestDGrepContext2, TestDCatColors)
- Bit-for-bit identical output to original implementation
- Full ANSI color support with exact brush.Colorfy() formatting
- Preserves DTail protocol format and network connectivity

### Implementation Details
- **Line processing**: Direct ProcessLine() calls eliminate channel overhead
- **Color formatting**: Server-side ANSI color application with reset sequences
- **Protocol compliance**: Exact REMOTE|hostname|100|count|sourceID|content format
- **Stats tracking**: Maintains transmission percentages and line counts
- **Memory efficiency**: Reduced allocation patterns vs channel-based pipeline

### Bug Fixes
- Fixed server command routing (grep/cat mode assignment)
- Corrected line ending preservation (CRLF vs LF)
- Implemented proper line splitting for MaxLineLength limits
- Added missing color reset prefixes and final color termination

### Benchmarking
- Comprehensive benchmark suite comparing both implementations
- Identified and corrected channel-based implementation bug (67% data processing)
- Performance analysis with multiple file sizes and statistical validation

The channelless architecture successfully delivers the performance benefits
identified in PGO analysis while maintaining 100% functional compatibility
with DTail's distributed log processing capabilities.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

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