Add Within() function with three signatures:
- Within(locator) to begin scoped context
- Within() to end current context
- Within(locator, fn) callback pattern (existing behavior)

Lowercase within() kept as deprecated alias with one-time warning.
switchTo() in helpers auto-ends any active Within context.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

…ner test

Add back @param/@return JSDoc annotations on the deprecated within()
wrapper so jsdoc generates the TypeScript definition (fixes dtslint).

Filter [deprecated] lines in within runner test so the deprecation
warning doesn't break step order assertion.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

…ontext control

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>