feat: add Init function (all but last)

[rjNemo]

Summary

• Add Init: returns all elements except the last one, and the last element separately
• Useful for list destructuring from the right

Implementation

• init.go: Init implementation with proper memory allocation
• Comprehensive tests including edge cases (empty, single, two elements)
• Non-mutating: returned slice is a copy
• Benchmark included

Testing

go test -v -run TestInit
go test -bench=BenchmarkInit -benchmem

All tests pass.

Examples

Init([]int{1,2,3,4,5})    // ([1,2,3,4], 5)
Init([]int{42})           // ([], 42)
Init([]int{})             // ([], 0)
Init([]string{"a","b"})   // (["a"], "b")

Related

Resolves Issue 17 from ACTION_PLAN.md

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.67%. Comparing base (d622c8c) to head (3df26bb).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #45      +/-   ##
==========================================
+ Coverage   98.63%   98.67%   +0.03%     
==========================================
  Files          39       40       +1     
  Lines         367      376       +9     
==========================================
+ Hits          362      371       +9     
  Misses          4        4              
  Partials        1        1              

Flag	Coverage Δ
unittests	98.67% <100.00%> (+0.03%)	⬆️

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