feat: migrate from bincode to wincode serialization

[dotslashapaar]

Work in progress #120

Replace unmaintained bincode (RUSTSEC-2025-0141) with wincode across the
codebase. Implements feature flag wincode for safe rollout with byte-level
wire format compatibility.

Changes:

• Add wincode_schema.rs (~1200 lines) with custom SchemaWrite/SchemaRead
  implementations for Solana transaction types
• Implement ShortVec wrapper for compact-u16 length encoding (1-3 bytes)
• Add manual VersionedMessageSchema handling bit-masked discriminator (0x80)
• Feature-gate all serialization call sites in payload.rs, solana.rs,
  grpc_jet.rs, pubkey_challenger.rs, transaction_handler.rs, and gentx.rs
• Add 19 wire compatibility tests verifying bincode/wincode equivalence
• Add OneTimeAuthTokenSchema roundtrip test for comprehensive coverage

Wire format remains identical to bincode for backward compatibility with
existing clients. Feature disabled by default; enable with --features wincode.