fix: resolve lint violations blocking xo v4 / ava v8 upgrade

[mstuart]

Root cause

The open dependabot PR (#6) bumping xo (1.2.3 -> 4.0.0) and ava (7.0.0 -> 8.0.1) has been failing CI on all Node versions, which is why the automated merge daemon has been skipping it (and would skip any future grouped dependency-update PR touching xo) for weeks.

xo 4.0.0 pulls in newer eslint-plugin-unicorn/eslint-plugin-jsdoc rules that flag pre-existing patterns already present in index.js/index.d.ts on master. Because package.json pins xo to ^1.2.3, master's own CI never sees these rules and stays green — but the moment any PR bumps xo past that range, the new rules fire against unrelated, already-merged code:

• unicorn/no-duplicate-if-branches x2 — the array/object branches of diff() had identical bodies
• unicorn/no-computed-property-existence-check x2 — key in object checks
• unicorn/prefer-continue — a whole loop body wrapped in one if
• unicorn/no-break-in-nested-loop x3 — break inside a switch nested in patch()'s for loop
• jsdoc/informative-docs — a @param description that just repeated the param name

Fix

Refactored the flagged spots without changing behavior:

• Merged the duplicate object/array branches in diff() into a single condition (they already called the same recursive diff() regardless of array vs. object).
• Replaced key in object with Object.hasOwn(object, key).
• Converted the removal-pass if into an early continue.
• Extracted patch()'s switch into a standalone applyOperation() helper so its break statements are no longer nested inside the outer loop.
• Reworded the repetitive JSDoc description.

This is a source-only fix — package.json is untouched, since the actual dependency bump belongs to the dependabot PR, not this one.

Verification

• npm test (xo && ava && tsd) passes clean against the currently pinned versions (xo@1.2.3, ava@7.0.0) — only a pre-existing, non-blocking complexity warning remains.
• Also verified against the exact versions the open dependabot PR bumps to (xo@4.0.0, ava@8.0.1): all 9 previously-failing lint errors are gone, all 34 ava tests pass unmodified, tsd passes.

Note (separate issue, not touched here)

.github/workflows/release.yml triggers push: branches: [main], but this repo's default branch is master — so the release workflow never runs. Flagging for awareness; out of scope for this CI fix.