feat: add equated monthly installments to financial

[AliAlimohammadi]

Description

Adds an implementation of the Equated Monthly Installment (EMI) calculation to the financial module. EMI is the fixed monthly payment a borrower makes to fully repay a loan over a given term, covering both principal and interest.

The formula used is:

$$A = \frac{p \cdot r \cdot (1 + r)^n}{(1 + r)^n - 1}$$

where $p$ is the principal, $r$ is the monthly interest rate (annual rate ÷ 12), and $n$ is the total number of monthly payments (years × 12). A zero-rate edge case is handled separately to avoid division by zero, returning principal / n directly.

Reference: Equated Monthly Installment — Wikipedia

Checklist:

• I ran cargo clippy --all -- -D warnings just before my last commit and fixed any issue that was found.
• I ran cargo fmt just before my last commit.
• I ran cargo test just before my last commit and all tests passed.
• I added my algorithm to the corresponding mod.rs file within its own folder, and in any parent folder(s).
• I added my algorithm to DIRECTORY.md with the correct link.
• I checked CONTRIBUTING.md and my code follows its guidelines.

[AliAlimohammadi]

@siriak, this is ready to be merged.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.01%. Comparing base (39c6978) to head (b486dbe).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1020   +/-   ##
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  Coverage   96.01%   96.01%           
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  Files         383      384    +1     
  Lines       28852    28890   +38     
=======================================
+ Hits        27701    27738   +37     
- Misses       1151     1152    +1     

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[siriak]

Looks good, thanks!