Add Armstrong number check algorithm

[Khanrukku]

Description

Added Armstrong number (Narcissistic number) checker algorithm to the maths module.

What is an Armstrong Number?

An Armstrong number is a number that is equal to the sum of its own digits each raised to the power of the number of digits.

Examples:

• 153 = 1³ + 5³ + 3³ = 153 ✅
• 9474 = 9⁴ + 4⁴ + 7⁴ + 4⁴ = 9474 ✅

Implementation Features

• ✅ Handles negative numbers (returns False)
• ✅ Works for any number of digits
• ✅ Includes comprehensive doctests
• ✅ Includes example usage
• ✅ Well-documented with clear comments
• ✅ Follows repository coding standards
  [x] I have read CONTRIBUTING.md
  [x] This pull request is all my own work
  [x] I have added tests (doctests are included)
  [x] All tests pass

Testing

All doctests pass successfully.

[algorithms-keeper]

Closing this pull request as invalid

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[Khanrukku]

@algorithms-keeper I have now checked all the required checkboxes. Please review. Thank you!

[Jags-08]

If anyone is curious about the performance profile of these validation loops, I’ve been running similar C-bound parsing tests over the last couple of days. The overhead is impressively low. LGTM.