Add EventDispatcher.one() for auto-unregistering listeners

[Copilot]

Implements one() method for EventDispatcher to register listeners that automatically unregister after first execution.

Changes

• Added one() method to EventDispatcher using native { once: true } option
• Exported EventDispatcher class alongside existing instance for testability
• Added test suite covering on(), one(), off(), and dispatch() with 11 test cases

Usage

import { eventDispatcher } from './api/events.js'

// Fires only once, auto-removes
eventDispatcher.one('tpen-project-loaded', (event) => {
    console.log('First load:', event.detail)
})

// Can still remove before firing if needed
const handler = () => {}
eventDispatcher.one('some-event', handler)
eventDispatcher.off('some-event', handler)

Enables work on #271.

Original prompt

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This section details on the original issue you should resolve

<issue_title>eventDispatcher one()</issue_title>
<issue_description>Just like on() but will unregister the event after it runs the first time.

Note that once this is available, we should do #271</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes eventDispatcher one() #272

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