Add @stdlib/stats/incr/nanmmaape - Moving MAAPE with NaN handling

Implement moving mean arctangent absolute percentage error with NaN handling. Add thin wrapper around mmaape that ignores NaN values in either the forecast or actual argument. Include comprehensive test suite (52 tests, all passing), TypeScript definitions and type tests, benchmarks for performance testing, examples demonstrating NaN handling, and complete documentation in README.

Closes RFC for nanmmaape implementation.

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Resolves #5562

Description

What is the purpose of this pull request?

This pull request:

• Adds a new package @stdlib/stats/incr/nanmmaape to compute a moving mean arctangent absolute percentage error (MAAPE) incrementally, ignoring NaN values
• Implements a thin wrapper pattern around @stdlib/stats/incr/mmaape that skips NaN values in either the forecast (f) or actual (a) parameters
• When NaN values are provided, they are ignored and the window state remains unchanged, preventing NaN propagation through the moving window
• Includes comprehensive test coverage (52 tests) covering basic functionality, NaN handling in all argument combinations, window sliding behavior, and edge cases
• Provides TypeScript type definitions and type tests for proper type safety
• Includes performance benchmarks demonstrating ~10M ops/sec for normal operation and ~7.5M ops/sec with NaN values
• Contains working examples showing real-world usage with random NaN injection
• Features complete API documentation in README with usage examples, mathematical equations, and notes on behavior

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• #{{TODO: add RFC issue number for nanmmaape}}

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Implementation Notes:

• The package follows the established pattern of @stdlib/stats/incr/nansum wrapping @stdlib/stats/incr/sum
• Key behavior: When NaN is detected in either argument, the underlying mmaape accumulator is not called, so the window state remains unchanged
• All 52 tests pass, covering comprehensive scenarios including mixed NaN/non-NaN values
• All 9 benchmarks pass successfully
• Examples run successfully demonstrating proper NaN handling

Test Results:
✅ Total: 52 tests
✅ Passing: 52
✅ Failing: 0
✅ Duration: 43-55ms

Benchmark Results:
✅ Constructor: ~21M ops/sec
✅ Accumulator: ~10M ops/sec
✅ Accumulator with NaN: ~7.5M ops/sec

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

AI Assistance

When authoring the changes proposed in this PR, did you use any kind of AI assistance?

• Yes
• No

If you answered "yes" above, how did you use AI assistance?

• Code generation (e.g., when writing an implementation or fixing a bug)
• Test/benchmark generation
• Documentation (including examples)
• Research and understanding

Disclosure

If you answered "yes" to using AI assistance, please provide a short disclosure indicating how you used AI assistance. This helps reviewers determine how much scrutiny to apply when reviewing your contribution. Example disclosures: "This PR was written primarily by Claude Code." or "I consulted ChatGPT to understand the codebase, but the proposed changes were fully authored manually by myself.".

This PR was authored with significant AI assistance (GitHub Copilot). The AI helped with:

• Studying reference packages (mmaape, nansum, sum) to understand implementation patterns
• Generating the thin wrapper implementation following the nansum pattern
• Creating comprehensive test suite adapted from mmaape tests with NaN-specific scenarios
• Writing TypeScript definitions and type tests
• Generating benchmarks with NaN handling scenarios
• Producing complete documentation in README with proper formatting

All code follows stdlib conventions and passes all static analysis checks. The implementation was iteratively tested and refined to ensure correct behavior and comprehensive test coverage.

@stdlib-js/reviewers