Add semantic compression module

[Siddhant-K-code]

Implements the semantic compression module as specified in #1.

Summary

Adds pkg/compress with three compression strategies to achieve 3-5× additional reduction on top of deduplication.

Components

• Extractive (extractive.go) - Selects salient spans using sentence-level scoring based on position, length, and content signals
• Placeholder (placeholder.go) - Compresses JSON/XML/tables to compact summaries while preserving key fields
• Pruner (pruner.go) - Removes filler phrases ("as mentioned earlier", "basically", etc.) and redundant patterns

API

type Compressor interface {
    Compress(ctx context.Context, chunks []types.Chunk, opts Options) ([]types.Chunk, Stats, error)
}

// Pipeline chains multiple strategies
pipeline := compress.NewPipeline(
    compress.NewPruner(),
    compress.NewExtractiveCompressor(),
)

Files

pkg/compress/
├── compress.go        # Interface, Options, Stats, Pipeline
├── extractive.go      # Salient span extraction
├── placeholder.go     # Structured content compression
├── pruner.go          # Filler phrase removal
└── compress_test.go   # Unit tests

Testing

Includes comprehensive tests for all compressors and the pipeline.

Closes #1