Fix DBN v3 instrument definition layout

[dawsh2]

Summary

• Decode and encode DBN v3 InstrumentDefMsg using the fixed binary field layout instead of text encode_order.
• Use 71-byte DBN v2/v3 symbol fields for raw_symbol and leg_raw_symbol.
• Align InstrumentClass enum values with Databento DBN values (CALL = 'C', PUT = 'P', STOCK = 'K', etc.).
• Model v3 leg ratio fields as signed Int32.
• Add a byte-offset regression test for v3 InstrumentDefMsg layout so round-trip tests do not hide a shared reader/writer layout bug.

Validation

• julia --project=/Users/daws/repos/DatabentoBinaryEncoding.jl -e 'using Pkg; Pkg.test()' => 3520/3520 passed.
• Decoded real DBN v3 definition files successfully:
  • option definitions: 5290 records, first raw symbol SPY260618C00390000, class CALL.
  • spot definitions: 1 record, raw symbol SPY, class STOCK.
• Local record-body parity check against the Databento-maintained DBN implementation: Rust and Julia emitted identical 520-byte v3 InstrumentDefMsg record bytes.

[tbeason]

Thanks for the contribution!