[profit] calculate_profit returns anyhow::Error — structured ProfitError thiserror enum required

[obchain]

Refs #40

File: crates/charon-core/src/profit.rs

PR branch: feat/15-profit-calc-and-queue

Problem:
calculate_profit() returns anyhow::Result. The executor consuming this function must distinguish:

• BelowMinThreshold: opportunity rejected on economics; log and skip to next entry.
• InvalidInputs / Overflow / InvalidBps: programming or data error; abort cycle and alert.

With anyhow::Error both cases collapse to a string. The executor cannot match on error variant to decide safe recovery behavior. This is the same defect flagged as p1 in the PR #39 review (FlashLoanProvider) and the pattern established in the PR #28 review (LendingProtocol trait).

Additionally, thiserror is not in [workspace.dependencies] or charon-core/Cargo.toml. Adding it is a required Cargo.toml change not present in this PR.

Fix:

1. Add thiserror = "1" to [workspace.dependencies] in root Cargo.toml.
2. Add thiserror to charon-core/Cargo.toml [dependencies].
3. Define:
   #[derive(Debug, thiserror::Error)]
   #[non_exhaustive]
   pub enum ProfitError {
   #[error("net {net_cents} cents below minimum {min_cents} cents")]
   BelowMinThreshold { net_cents: u64, min_cents: u64 },
   #[error("invalid bps {bps}: must be <= 10_000")]
   InvalidBps { bps: u32 },
   #[error("arithmetic overflow")]
   Overflow,
   #[error("invalid price input: {reason}")]
   InvalidPrice { reason: &'static str },
   }
4. Change calculate_profit signature to -> Result<NetProfit, ProfitError>.

[obchain]

Resolved in f8f01fb on feat/15-profit-calc-and-queue.