chore: adopt oxlint as lint gate

[ElbertePlinio]

No JS linter today (tsc + vitest + playwright). Solid app, 248 files. oxlint has no eslint-plugin-solid equivalent, so reactivity rules stay uncovered (they are uncovered today too). Effort: M.

Baseline: 29 findings — 20 no-unassigned-vars (almost all Solid ref bindings: let root; <div ref={root}> — false positives), 6 no-unused-expressions, 1 each prefer-string-starts-ends-with, no-irregular-whitespace, no-control-regex.

Agent plan

Goal: oxlint is the primary lint gate (edit loop + CI), per workspace AGENTS.md "Frontend tooling". Audit source: oxlint 1.80.0 run on this tree, 2026-08-25.

Checklist:

• Add oxlint + .oxlintrc.json
• Handle Solid ref false positives: prefer let root!: HTMLDivElement or disable no-unassigned-vars for .tsx
• Fix the 9 real findings
• Add CI Lint step
• lint script runs oxlint; CI "Lint" step uses it
• Baseline is clean (fix findings or justify per-rule disables in .oxlintrc.json)
• Add .complexity-gate.json (complexity-gate init, defaults) and complexity-gate check <src> in CI; existing FAILs are legacy — gate only changed functions via hooks, but CI baseline must be clean or explicitly allowlisted
• PR/review

Validation:

• bunx oxlint (or npx oxlint) exits 0 on the tree
• complexity-gate check <src> exits 0
• CI green

Current status: Planned
Next action: open PR

[ElbertePlinio]

Plan updated: added complexity-gate alongside oxlint. They are complementary — oxlint = correctness rules, complexity-gate = structural limits (complexity/depth/lines/params, supports TS/TSX/Svelte/Rust/Dart). Not either/or.