Align the Platform contract with the spec port table

[zealsprince]

The prototype already has most of this: the contract lives in packages/platform/src/types.ts and the web adapter covers notifications, audio enumeration, and file download. Two gaps against the spec. The contract marks notifications, audioDevices, and fileTransfer nullable where the spec's interface has every target providing them, and no test pins the contract, so a port can drift without anything failing.

Tighten the interface so null strictly means "this target can't do this". Then write a mock adapter with plain stubs, like the spec's Testability example, and a headless test that runs app code against it with no DOM involved. Last, move the per-target port mapping into the contract as doc comments, so someone reading types.ts can tell designed-null from not-built-yet without opening the spec.

Spec

• Platform - the port table and the Testability mock example

Acceptance criteria

• notifications, audioDevices, and fileTransfer are non-nullable in the contract; only tray, deepLinks, and dns stay nullable, matching the spec interface
• A mock Platform adapter exists and a headless test exercises app code through it
• Every port in types.ts has a doc comment naming which targets provide it and where null is by design

Out of scope

• The historyCache port (Clients: local history cache #10 adds it)
• Desktop implementations (own ticket)

No blockers.

[dolanske]

I noticed one inconsistency in the spec. The Platform interface defines tray as Trayport | null, but the following port table mentions both document.title and favicon overlay - making Tray non-nullable as well, because all targets can implement it to some extent.

Currently on my branch, I've extended the TrayPort interface with new methods

export interface TrayPort {
  /**
   * Sets the red badge count on the favicon/app icon.
   */
  setBadgeCount: (count: number) => Promise<void>
  /**
   * Add/Remove an alert circle from the document/tray icon.
   */
  addBadgeAlert: () => Promise<void>
  /**
   * Remove any active badge from the icon.
   */
  removeBadge: () => Promise<void>
  /**
   * Sets the application title.
   *
   * - on web it's the document.title
   * - on native it's the title of the tooltip which appears on hover over the app icon
   * - on mobile this is a noop
   */
  setTitle: (title: string) => Promise<void>
}