feat: magicless frame format support

[polaz]

Summary

The C reference supports ZSTD_f_zstd1_magicless format — frames without the 4-byte magic number prefix. Used in embedded protocols where the frame format is known from context, saving 4 bytes per frame.

C reference

• ZSTD_f_zstd1_magicless format variant
• Encoder: skip magic number write
• Decoder: ZSTD_d_format parameter to expect magicless frames

Current Rust state

• Encoder always writes magic number
• Decoder always expects magic number

What needs to be implemented

1. Encoder option to skip magic number
2. Decoder option to expect magicless frames
3. API — format parameter on both encoder and decoder

Acceptance criteria

• Magicless encode/decode roundtrip
• C zstd interop in magicless mode

Time estimate

4h