Host weight format header parser (R-HS-14)

[gHashTag]

Problem

No structured header for weight files — need magic, version, dimensions, flags, checksum seed.

Solution

• WeightHeader — 16-byte binary header (magic 0x54325700, version, layers, neurons_per_layer, bits_per_weight, flags, checksum_seed)
• Builder pattern: WeightHeader::new(layers, neurons).with_checksum_seed(s).with_bits_per_weight(b)
• encode() → [u8; 16], decode(&[u8]) → Result<WeightHeader, HeaderError>
• Validation: bad magic, unsupported version, zero dimensions, invalid bits_per_weight
• total_weights(), total_weight_bytes(), has_crc(), is_ternary()
• 15 new tests, all pass.