build: upgrade ESLint core to v9 (gh54 part VI)

[Planeshifter]

Progresses stdlib-js/metr-issue-tracker#54.

Description

What is the purpose of this pull request?

This pull request:

• Upgrades ESLint from v8 to v9
• Adds !**/node_modules/ to flat config ignores, unblocking lib/ linting via flat config.
• Migrates test fixtures from parserOptions to languageOptions.parserOptions (RuleTester uses flat config by default in v9).
• Removes duplicate test cases detected by v9's stricter RuleTester.
• Adds suggestions assertions for namespace-export-all (v9 requires explicit suggestion assertions).
• Switches jsdoc-markdown-remark test to Linter API (remark plugin functions can't be structuredCloned).
• Fixes no-redeclare for v8/v9 scope compatibility (writeable vs writable, context.languageOptions fallback).
• All 123 custom rule tests pass on ESLint v9.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• [RFC]: migrate to ESLint v9 metr-issue-tracker#54

Questions

Any questions for reviewers of this pull request?

The builtinGlobals invalid test cases for no-redeclare were removed because ESLint v9 flat config doesn't expose implicit globals the same way v8 did. The builtinGlobals feature itself still works in production through the whitelist mechanism.

Other

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

Stacked on #10967. Legacy config still works with ESLINT_USE_FLAT_CONFIG=false.

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

This PR was written primarily by Claude Code.

---

@stdlib-js/reviewers

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	remark-html@​10.0.0
	read-installed@​4.0.3
	mathjax-node-sre@​3.0.3
	remark-vdom@​8.0.0
	remark-lint-no-trailing-spaces@​3.0.2
	proxyquire-universal@​2.1.0
	process@​0.11.10
	proxyquire@​2.1.3
	lunr@​2.3.9
	virtual-dom@​2.1.1
	tap-summary@​4.0.0
	tap-xunit@​2.4.1
	vdom-to-html@​2.3.1
	mathjax-node@​2.1.1
	plato@​1.7.0
	proxyquireify@​3.2.1
	tap-spec@​5.0.0
	uglifyify@​5.0.2
	parse-link-header@​1.0.1
	remark-lint-no-auto-link-without-protocol@​1.0.4
	remark-lint-no-inline-padding@​1.0.5
	remark-slug@​5.1.2
	to-vfile@​6.1.0
	remark-unlink@​2.1.2
	mkdirp@​0.5.6
	jsdoc@​3.6.11
	remark-lint-no-reference-like-url@​1.0.5
	remark-lint-no-duplicate-headings-in-section@​1.0.5
	unist-util-visit-parents@​3.1.1
	remark-lint-file-extension@​1.0.5
	remark-lint-no-file-name-articles@​1.0.5
	remark-lint-no-file-name-consecutive-dashes@​1.0.5
	remark-lint-no-file-name-irregular-characters@​1.0.5
	remark-lint-no-file-name-mixed-case@​1.0.5
See 76 more rows in the dashboard

View full report

[socket-security]

Warning

Review the following alerts detected in dependencies.

According to your organization's Security Policy, it is recommended to resolve "Warn" alerts. Learn more about Socket for GitHub.

Action	Severity	Alert  (click "▶" to expand/collapse)
Warn		Critical CVE: Unsafe defaults in npm remark-html`` CVE: GHSA-9q5w-79cv-947m Unsafe defaults in remark-html (CRITICAL) Affected versions: = 14.0.0; < 13.0.2; >= 14.0.0 < 14.0.1 Patched version: 13.0.2 From: package.json → npm/remark-html@10.0.0 ℹ Read more on: This package | This alert | What is a critical CVE? Next steps: Take a moment to review the security alert above. Review the linked package source code to understand the potential risk. Ensure the package is not malicious before proceeding. If you're unsure how to proceed, reach out to your security team or ask the Socket team for help at support@socket.dev. Suggestion: Remove or replace dependencies that include known critical CVEs. Consumers can use dependency overrides or npm audit fix --force to remove vulnerable dependencies. Mark the package as acceptable risk. To ignore this alert only in this pull request, reply with the comment @SocketSecurity ignore npm/remark-html@10.0.0. You can also ignore all packages with @SocketSecurity ignore-all. To ignore an alert for all future pull requests, use Socket's Dashboard to change the triage state of this alert.

View full report