feat: add First and FirstN functions

[rjNemo]

Summary

• Add First: returns first element with error handling
• Add FirstN: returns first n elements safely

Implementation

• first.go: First and FirstN implementations with proper error handling
• New ErrEmptySlice error for consistent error handling
• Comprehensive tests including edge cases (empty, negative n, n > length)
• Benchmarks included

Testing

go test -v -run "TestFirst"
go test -bench="BenchmarkFirst" -benchmem

All tests pass.

Examples

// First
val, err := First([]int{1,2,3})  // val=1, err=nil
val, err := First([]int{})       // val=0, err=ErrEmptySlice

// FirstN
FirstN([]int{1,2,3,4,5}, 3)   // [1,2,3]
FirstN([]int{1,2,3}, 10)      // [1,2,3]
FirstN([]int{1,2,3}, 0)       // []
FirstN([]int{1,2,3}, -1)      // []

Related

Resolves Issue 16 from ACTION_PLAN.md

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.21%. Comparing base (d622c8c) to head (8031d77).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #44      +/-   ##
==========================================
+ Coverage   98.63%   99.21%   +0.57%     
==========================================
  Files          39       40       +1     
  Lines         367      382      +15     
==========================================
+ Hits          362      379      +17     
+ Misses          4        2       -2     
  Partials        1        1              

Flag	Coverage Δ
unittests	99.21% <100.00%> (+0.57%)	⬆️

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