[RFC]: Tracking issue for complete implementation of `@stdlib/stats/base/dists/halfnormal` package

### Description

This tracking issue proposes adding comprehensive support for the half-normal distribution in `@stdlib/stats/base/dists/halfnormal`. The half-normal distribution is the distribution of the absolute value of a normally distributed random variable with mean 0. It is commonly used in statistical applications such as stochastic frontier analysis, modeling measurement errors, and other fields where only the magnitude of a normally distributed variable is of interest.

## Rationale

**Why is this feature important?**

1. **Completeness**: Expanding stdlib's statistical distributions library to include the half-normal distribution provides users with a comprehensive set of tools for statistical analysis.

2. **Common Use Cases**: The half-normal distribution is widely used in:
- Stochastic frontier analysis
- Quality control and process capability studies
- Modeling absolute deviations and measurement errors
- Bayesian analysis as a prior distribution

3. **Consistency**: Following stdlib's existing patterns for probability distributions ensures a consistent API and user experience.

4. **Scientific Computing**: Enables researchers and data scientists to perform half-normal distribution calculations natively within stdlib without external dependencies.

## Proposed Solution

**How do you propose to implement this feature?**

Implement the half-normal distribution following the established pattern used for other continuous distributions in stdlib (e.g., normal, exponential, gamma). This includes creating subpackages for various distribution functions, properties, and utilities within `@stdlib/stats/base/dists/halfnormal/`.

## Implementation Checklist

The following subpackages need to be implemented for the half-normal distribution:

### Core Distribution Functions

- [ ] **`@stdlib/stats/base/dists/halfnormal/cdf`** - Cumulative distribution function (CDF)
- [ ] **`@stdlib/stats/base/dists/halfnormal/pdf`** - Probability density function (PDF)
- [ ] **`@stdlib/stats/base/dists/halfnormal/quantile`** - Quantile function (inverse CDF)
- [ ] **`@stdlib/stats/base/dists/halfnormal/logcdf`** - Logarithm of cumulative distribution function
- [ ] **`@stdlib/stats/base/dists/halfnormal/logpdf`** - Logarithm of probability density function

### Distribution Properties

- [ ] **`@stdlib/stats/base/dists/halfnormal/mean`** - Expected value (mean)
- [ ] **`@stdlib/stats/base/dists/halfnormal/median`** - Median
- [ ] **`@stdlib/stats/base/dists/halfnormal/mode`** - Mode
- [ ] **`@stdlib/stats/base/dists/halfnormal/variance`** - Variance
- [ ] **`@stdlib/stats/base/dists/halfnormal/stdev`** - Standard deviation
- [ ] **`@stdlib/stats/base/dists/halfnormal/entropy`** - Differential entropy
- [ ] **`@stdlib/stats/base/dists/halfnormal/skewness`** - Skewness
- [ ] **`@stdlib/stats/base/dists/halfnormal/kurtosis`** - Excess kurtosis

### Generating Functions

- [ ] **`@stdlib/stats/base/dists/halfnormal/mgf`** - Moment-generating function (MGF)

### Constructor and Utilities

- [ ] **`@stdlib/stats/base/dists/halfnormal/ctor`** - Distribution constructor

### Main Package

- [ ] **`@stdlib/stats/base/dists/halfnormal`** - Main package with exports and documentation

### Related Issues

- #8205 - Original RFC for half-normal distribution support
- #6951 - Related issue mentioned in #8205

### Questions

- Are there any additional distribution-specific functions that should be included beyond the standard set?
- Should we implement any specialized constants related to the half-normal distribution?

### Other

No.

### Checklist

- [x] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [x] Searched for existing issues and pull requests.
- [x] The issue name begins with `RFC:`.

Start by reviewing the existing normal, exponential, and gamma distribution patterns in @stdlib/stats/base/dists, then read related issues #8205 and #6951. Done means implementing the listed halfnormal subpackages, wiring the main package exports and documentation, and resolving the open questions about additional functions and constants.

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