Leave ESLint 8: run oxlint first and keep a slim flat-config ESLint for what it lacks

[Pixnop]

Summary

ESLint 8 has been end of life since 2024-10-05 and the repository still uses the legacy .eslintrc.cjs, so a migration is due whatever we choose. Measured on dev, oxlint covers most of what we lint for at about a twentieth of the time, but not everything. Proposal: oxlint runs first, a slimmed ESLint 9 (flat config) keeps the rules oxlint does not enforce for us, wired with eslint-plugin-oxlint so nothing is checked twice.

Measurements (dev, Node 22, median of three runs)

	Time	Findings
npm run lint:ci (ESLint 8.57)	10.2 s	0 errors, 14 warnings, all react-hooks/exhaustive-deps
oxlint 1.83.0 with the react plugin	0.53 s	the same violations in the same 13 files (reported as 15, one split in two)

Parity

• react-hooks/exhaustive-deps: implemented, on by default. rules-of-hooks: implemented, off by default (category "suspicious"), to switch on.
• The typescript-eslint rules our config sets to error (no-explicit-any, ban-ts-comment, explicit-function-return-type, no-empty-function, no-empty-interface) and the no-restricted-imports guard that keeps src/domain pure all exist in oxlint but are off by default: each has to be enabled explicitly and proven with a failing fixture.
• Type-aware linting needs the separate oxlint-tsgolint package, still experimental: not adopted here.
• @oxlint/migrate only ported two rules and ignored every extends and both overrides blocks, so the config is written by hand.

What oxlint found that ESLint did not

• Real: no-unsafe-optional-chaining in four test files (genuine), react/set-state-in-effect in 23 places and react/refs in 14, to triage rather than fix blindly.
• Style: four unicorn hits.
• Noise: two false positives on src/global.d.ts that tsc accepts.

Plan

1. .oxlintrc.json written by hand: the rules above switched on, the two global.d.ts false positives ignored with a comment, a fixture test proving the domain import guard and rules-of-hooks fire.
2. ESLint moved to 9 with a flat config reduced to what oxlint does not cover, plus eslint-plugin-oxlint.
3. lint and lint:ci run oxlint then ESLint; CI unchanged otherwise. Prettier is separate and unaffected.
4. The new real findings triaged in their own issue, not mixed into the migration.

Acceptance

• The 14 known warnings are still reported, no rule we rely on is lost (one fixture per guard).
• lint:ci wall time reported before and after.
• No ESLint 8 package left in package.json.

[Zaldaryon]

Resolved in pull request #522 (merge commit 021badc). Migrates the repository from ESLint 8 to ESLint 9 flat configuration in eslint.config.mjs with oxlint as a fast first-pass linter, retiring .eslintrc.cjs and .eslintignore while maintaining all domain boundary and react-hooks rules.