feat: add Scan function (reduce with history)

[rjNemo]

Summary

• Add Scan: returns all intermediate accumulator values during reduction
• Also known as prefix scan or cumulative fold

Implementation

• scan.go: Scan implementation with proper memory pre-allocation
• Comprehensive tests including edge cases, different operations, and type conversions
• Benchmark included

Testing

go test -v -run TestScan
go test -bench=BenchmarkScan -benchmem

All tests pass.

Examples

// Running sum
Scan([]int{1,2,3,4}, 0, func(acc, n int) int { return acc + n })
// → [1, 3, 6, 10]

// Running max
Scan([]int{3,1,4,1,5}, 0, func(acc, n int) int {
    if n > acc { return n }
    return acc
})
// → [3, 3, 4, 4, 5]

// String concatenation
Scan([]string{"a", "b", "c"}, "", func(acc, s string) string { return acc + s })
// → ["a", "ab", "abc"]

Related

Resolves Issue 15 from ACTION_PLAN.md

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.66%. Comparing base (d622c8c) to head (5c8e892).
⚠️ Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #43      +/-   ##
==========================================
+ Coverage   98.63%   98.66%   +0.02%     
==========================================
  Files          39       40       +1     
  Lines         367      375       +8     
==========================================
+ Hits          362      370       +8     
  Misses          4        4              
  Partials        1        1              

Flag	Coverage Δ
unittests	98.66% <100.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.