Audit #[repr(u8)], #[derive] order, and serde conventions

[joaquinbejar]

Overview

Audit #[repr], #[derive] order, and serde attributes across every
public type. Small stable enums get #[repr(u8)]; #[derive] clauses
follow the canonical order; single-field newtypes use
#[serde(transparent)]; public-facing enums use
#[serde(rename_all = "snake_case")] unless an existing wire contract
prevents it; deserialized config / input DTOs use
#[serde(deny_unknown_fields)].

Context

• rules/global_rules.md §Compiler Attributes — §#[repr], §#[derive],
  §Serde conventions.
• Canonical derive order:
  Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord, Hash, Default.

Tasks

• Apply #[repr(u8)] to Side, OptionStyle, OptionType, and any
  other small stable enum (coordinate with upstream financial_types
  — avoid double-repr)
• Reorder every #[derive(...)] to the canonical sequence; strip
  unjustified Default / Ord / Copy
• #[serde(deny_unknown_fields)] on every input DTO
• #[serde(rename_all = "snake_case")] on public-facing enums unless
  an existing wire contract forbids it; document the exception

Acceptance criteria

• Canonical derive order everywhere
• DTOs deny unknown fields
• cargo clippy --all-targets --all-features --workspace -- -D warnings clean
• cargo test --all-features --workspace clean (serde round-trips)

References

• rules/global_rules.md — §Compiler Attributes