nodeApi: split mounted DOM/canvas widget lifecycle out of the widget collection

Follow-up owned from review of #16790 (thread on `widgetHandle.ts`).

## Context

`widgetHandle.ts` is ~1,200 lines and fuses two concerns in one factory: the ordered collection of widgets on a node (get, names, order-safe insertion, hide/show, linked widgets) and the lifecycle of a *mounted* widget — a DOM container or canvas strip the pack renders into, with its own change listeners, height allocation, and teardown.

The collection is graph data. The mount is renderer ownership with a disposal contract. They change for different reasons and under different renderers.

## Decision

Ticket rather than fix in #16790, for the same reason as #16853: a structural refactor inside the PR under review buys nothing a reviewer can check and risks the widget path, which is the most heavily used part of the surface.

## Done looks like

- The widget collection exposes ordering and lookup without knowing what a mount is
- Mount lifecycle — `render`, container ownership, `setHeight`, change subscription, disposal — lives in its own module with its own tests
- Teardown is provably tied to the mount that created it, not to a widget name
- No change to the published shapes; `widgets.mount()` and `widgets.canvas()` keep their contracts

Start with widgetHandle.ts and the review thread on #16790 to understand the existing collection and mounted-widget responsibilities. Separate collection ordering and lookup from mount rendering, ownership, height, change subscription, and disposal, then add module-specific tests. Done means teardown remains tied to its mount, published shapes are unchanged, and widgets.mount() and widgets.canvas() retain their contracts.

Written by the indexing model from the issue text.