feat: add Simple, Compound, and APR interest calculations

[AliAlimohammadi]

Description

Given a principal amount, an interest rate, and a time period, the module computes three flavours of interest return:

• Simple interest multiplies principal × daily rate × number of days directly:

$$I = p \times r \times t$$

• Compound interest applies the standard formula, where $r$ is the rate per compounding period and $n$ is the total number of periods:

$$I = p \times \left((1 + r)^n - 1\right)$$

• APR interest delegates to compound interest after converting the annual percentage rate to a daily rate and years to days, matching real-world lender disclosures:

$$I = p \times \left(\left(1 + \frac{r}{365}\right)^{t \times 365} - 1\right)$$

All three functions validate their inputs and return Result<f64, &'static str>, propagating a descriptive error string rather than panicking on bad input.

• Time complexity: $O(1)$
• Space complexity: $O(1)$

Type of change

• New feature (non-breaking change which adds functionality)

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.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.04%. Comparing base (a84b940) to head (f10329f).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1026      +/-   ##
==========================================
+ Coverage   96.02%   96.04%   +0.01%     
==========================================
  Files         386      386              
  Lines       29011    29102      +91     
==========================================
+ Hits        27858    27951      +93     
+ Misses       1153     1151       -2     

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[AliAlimohammadi]

@siriak, this is ready to be merged.