feat: dictionary builder — FastCOVER algorithm and dictionary finalization

[polaz]

Summary

The current dict_builder feature implements the COVER algorithm but lacks the FastCOVER variant and dictionary finalization (adding zstd header with entropy tables).

C reference implementation (lib/dictBuilder/)

FastCOVER (fastcover.c)

• Faster alternative to COVER using frequency array instead of suffix array
• Parameter f: frequency array size (0 < f ≤ 31)
• Parameter accel: acceleration factor (1-10, higher = faster, less accurate)
• Same output quality for ~10x less computation

Dictionary finalization (zdict.c)

• ZDICT_finalizeDictionary() — adds zstd header to raw content dictionary
• Header contains: magic number, Huffman table, FSE tables, offset history
• Result: full zstd dictionary format that enables entropy table sharing

Optimization functions

• ZDICT_optimizeTrainFromBuffer_cover() — grid search over k,d parameters
• ZDICT_optimizeTrainFromBuffer_fastCover() — same for FastCOVER
• Multi-threaded optimization via nbThreads parameter

Current Rust state

• dictionary/cover.rs — COVER algorithm implemented
• dictionary/frequency.rs and reservoir.rs — supporting structures
• No FastCOVER variant
• No dictionary finalization (only raw content output)
• No parameter optimization

What needs to be implemented

1. FastCOVER algorithm — frequency-based alternative to COVER
2. Dictionary finalization — add zstd header (magic + entropy tables + content)
3. Parameter optimization — grid search over k,d,f parameters
4. Configurable parameters — expose k, d, f, accel, splitPoint

Acceptance criteria

• FastCOVER produces dictionaries matching C quality
• Finalized dictionaries usable by C zstd for decompression
• Parameter optimization finds good k,d values automatically
• Performance: FastCOVER ≥5x faster than COVER

Time estimate

3d