chore(clippy): satisfy -D warnings on workspace lints

[obchain]

Workspace clippy is currently red under -D warnings for two reasons.

1. Cargo.toml:98 declares [workspace.lints.clippy] all = "warn" at default priority 0, colliding with specific denies. Newer clippy promotes lint-group-vs-specific-lint priority collisions to a stable lint, which -D warnings upgrades to error. Fix: all = { level = "warn", priority = -1 }.

2. The priority change unmasks 10 pre-existing errors:

   • crates/charon-core/src/config.rs:476 should_implement_trait (Config::from_str shadows FromStr::from_str)
   • crates/charon-core/src/config.rs:748,777 arithmetic_side_effects (checked_add)
   • crates/charon-core/src/profit.rs:65,199,242,276 result_large_err (Box ProfitError::Unprofitable)
   • crates/charon-core/src/profit.rs:213,288,350 arithmetic_side_effects on profit-path division (cosmetic vs non-zero constants)

Land as one PR after parallel-terminal work merges. Re-enable clippy in .github/workflows/test.yml once green.

[obchain]

Closed per maintainer direction. Lint debt acknowledged; will revisit if/when -D warnings is re-enabled in CI.