Add OKX Router Solana preset

[kyle-anchorage]

1. Why this PR exists

OKX Router (proVF4pMXVaYqmy4NjniPh4pqKNfMmsihgd4wdkCX3u) swaps on Solana fell through to the unknown-program catch-all visualizer. Signers saw raw instruction bytes instead of a decoded instruction name, named accounts, and arguments — making it hard to verify what they were signing.

This adds an IDL-backed preset so OKX Router instructions render as structured, human-readable fields.

2. What changed

• New Solana preset presets/okx_router/ with OkxRouterVisualizer, following the existing IDL-preset pattern (mirrors neutral_trade).
• Bundles the on-chain Anchor IDL (okx_router.json, ~3.5k lines) via include_str!; the visualizer parses instructions with parse_instruction_with_idl against the bundled IDL.
• Decodes the 8-byte Anchor discriminator, maps account positions to IDL account names, and renders instruction name, program ID, discriminator, named accounts, and decoded args in condensed/expanded preview layouts, with a raw-hex fallback.
• SolanaIntegrationConfig registers the program with a "*" -> "*" wildcard so all OKX Router instructions route to this visualizer.
• Registered by adding pub mod okx_router; to presets/mod.rs — the build.rs script auto-discovers the directory and wires OkxRouterVisualizer into available_visualizers().

3. Why this is safe

• Backward compatibility: Purely additive. No existing preset, type, or registration path is modified beyond one pub mod line. Before this, OKX Router txs hit the unknown-program catch-all; now they hit a dedicated visualizer. No other chain or program is affected.
• How it was built: Reuses the established InstructionVisualizer + SolanaIntegrationConfig + build.rs auto-discovery mechanism already used by every other Solana preset, so it inherits their behavior and routing.
• Risks mitigated: Instruction data shorter than 8 bytes and unknown discriminators return VisualSignError rather than panicking or mis-rendering. Decoding failures surface as errors; the field carries a raw-hex fallback so nothing is silently dropped.

Checks run (by agent)

• cargo test -p visualsign-solana okx — 4 passed (IDL loads, all discriminators are 8 bytes, unknown discriminator errors, short data errors)
• cargo clippy -p visualsign-solana --all-targets -- -D warnings — clean
• cargo fmt --check -p visualsign-solana — clean

Manual steps needed (by human)

None — fully automated by CI.

4. How this is maintainable

• Follows the exact same structure as other presets (neutral_trade, metadao_futarchy), so a fresh developer recognizes the pattern immediately. The solana-add-idl skill documents how to add more.
• Registration is automatic via build.rs directory scanning — no central registry to keep in sync; the directory name (okx_router) deterministically maps to OkxRouterVisualizer.
• Tests guard the IDL contract: that it loads, that every instruction has an 8-byte discriminator, and that malformed input errors rather than panics.

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