Feature/best vs ranked bid tests

[Samaro1]

This PR ensures best-bid selection always matches the first element of the ranked bid list, including all tie scenarios.

What changed:

Refactored bid selection logic to use a shared deterministic path for both best-bid and ranking flows.
Added comprehensive tie-case regression tests for:
equal profit with expected-return tie-break
equal profit/expected-return with bid-amount tie-break
equal economics with timestamp tie-break
full ties with bid_id final tie-break
insertion-order variance checks
Updated bid-ranking docs to reflect exact tie-break behavior and the best-vs-ranked invariant.
Security notes:

Centralized comparison path reduces drift risk between ranking and best-bid logic.
Deterministic ordering under full ties prevents inconsistent outcomes across validators.
Testing:

Attempted: cargo test test_bid_ranking -- --nocapture
Current status: blocked by unrelated pre-existing compile issues in the repository (src/lib.rs and src/invoice.rs).

Closes #709

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