style: align outliers in stats/base/dists/chi with namespace majority patterns

[Planeshifter]

Description

Aligning outliers in stats/base/dists/chi with namespace majority patterns (random namespace pick, seed 2026-04-22-drift).

Namespace summary

• Target namespace: @stdlib/stats/base/dists/chi
• Member count: 12 non-autogenerated packages (cdf, ctor, entropy, kurtosis, logpdf, mean, mode, pdf, quantile, skewness, stdev, variance)
• Features analyzed: file tree, package.json shape, README section layout, manifest.json shape, test/benchmark/examples naming, public signature, validation prologue, error construction, JSDoc shape, @stdlib/* dependencies
• Features with a clear majority (≥75% conformance): native-addon file set (binding.gyp, manifest.json, include/, src/, lib/native.js, benchmark/benchmark.native.js, test/test.native.js, examples/c/), top-level ## C APIs README section, validation prologue at top of lib/main.js, Julia fixture layout under test/fixtures/julia/
• Features without a clear majority (excluded): lib/factory.js (only present for functions taking a shape-parameter closure — 4/12) and related test.factory.js; these are structurally optional and excluded from drift detection

stats/base/dists/chi/stdev

Adds an explicit NaN / negative-k validation prologue to lib/main.js, aligning stdev with the convention followed by 10 of the 12 siblings in @stdlib/stats/base/dists/chi (~83% conformance). Previously, the guard was implicit — invalid inputs propagated through the variance delegation and happened to return NaN by accident rather than by contract. Observable behavior is unchanged; this is a correctness-by-construction fix that makes the invariant explicit at the call site.

stats/base/dists/chi/cdf

Restructures cdf/README.md to match the canonical section layout shared by 10 of 12 packages in the @stdlib/stats/base/dists/chi namespace (~83% conformance; reference: pdf/README.md, logpdf/README.md). The C APIs block is extracted from inside <section class="usage">, promoted to a top-level <section class="c"> wrapper, and its headings are raised one level (## C APIs, ### Usage). Section order is corrected to JS Usage → JS Examples → C APIs. No prose or code-block content is changed.

Related Issues

No.

Questions

No.

Other

Validation

• Structural features extracted via direct filesystem / package.json / README inspection across all 12 members.
• Semantic features (public signature, validation prologue, error construction, JSDoc shape, @stdlib/* dependencies) extracted per-package from lib/main.js.
• Three-agent validation (semantic, cross-reference, structural) run over each outlier finding; only findings confirmed by all applicable agents survived to this PR.

Deliberately excluded from this PR

• quantile/ lacks native C entirely (missing binding.gyp, manifest.json, include/, src/, lib/native.js, C benchmark/examples, test/test.native.js) — out of scope: adding native support requires authoring a C implementation, not a mechanical drift fix.
• quantile/lib/main.js lacks the majority validation prologue — cross-reference verdict was confirmed-drift, but the semantic-review agent flagged it as intentional-deviation because quantile is a one-liner that delegates to gammaQuantile (which performs NaN propagation and its own domain checks). Per the routine, any intentional-deviation verdict from any agent drops the correction.
• stdev/test/fixtures/ uses Python fixtures while 11/11 siblings (excluding ctor) use Julia fixtures — out of scope: regenerating fixtures from a different reference implementation is a cascading change that would require re-verifying tolerance against the existing test data.
• ctor/ outliers on virtually every structural feature — intentional deviation: ctor is a distribution constructor, not a scalar function, and has a genuinely different shape.

Checklist

• Read, understood, and followed the contributing guidelines.

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

This PR was authored by an automated cross-package drift-detection routine driven by Claude Code. The routine (1) selected stats/base/dists/chi at random (seed 2026-04-22-drift) from eligible stdlib namespaces (≥8 non-autogenerated members), (2) extracted structural and semantic features from every member, (3) computed majority patterns with a 75% conformance threshold, (4) ran three independent validation agents to confirm each outlier finding as mechanical, non-semantic drift, and (5) applied only corrections that preserved observable behavior and did not require cascading changes. Every edit and commit message was produced by the routine; a human maintainer should audit before promoting this PR out of draft.

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@stdlib-js/reviewers

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/chi/cdf	$\color{red}277/279$ $\color{green}+99.28%$	$\color{red}23/24$ $\color{green}+95.83%$	$\color{green}4/4$ $\color{green}+100.00%$	$\color{red}277/279$ $\color{green}+99.28%$
stats/base/dists/chi/stdev	$\color{green}163/163$ $\color{green}+100.00%$	$\color{green}8/8$ $\color{green}+100.00%$	$\color{green}2/2$ $\color{green}+100.00%$	$\color{green}163/163$ $\color{green}+100.00%$

The above coverage report was generated for the changes in this PR.