Closes #1987.

Problem

Passing a freshly created (parent-less) node as a child in a container constructor clones it, so the caller's original reference never gets a parent and later operations on it throw:

const decl = postcss.decl({ prop: 'foo', value: 'bar' })
postcss.atRule({ name: 'foo', nodes: [decl] })
decl.before(postcss.comment({ text: 'hello' }))
// TypeError: Cannot read properties of undefined (reading 'insertBefore')

The constructor unconditionally clones any real Node child:

if (typeof node.clone === 'function') {
  this.append(node.clone())
} else {
  this.append(node)
}

That clone was added in 9d19bce only to keep the source tree intact when moving nodes between two roots — but it also fires for brand-new, parent-less nodes that aren't part of any tree, where there's nothing to protect.

Fix

As you suggested on the issue, only clone when the node already belongs to another tree (node.parent is set); otherwise adopt the instance directly so append() sets its parent:

if (typeof node.clone === 'function' && node.parent) {
  this.append(node.clone())
} else {
  this.append(node)
}

Nodes that already have a parent (e.g. new Root({ nodes: root1.nodes })) are still cloned, so the move-between-trees behavior from 9d19bce is unchanged.

Tests

Adds a regression test (adopts root-less nodes in constructor instead of cloning them) that builds a container from a parent-less node and asserts the original instance is adopted (decl.parent is the new container, container.first === decl) and that operating on the original reference no longer throws. It fails on the current code and passes with this change. The two existing tests from 9d19bce (updates parent in overrides.nodes in constructor, allows to clone nodes) still pass — the full unit suite (653 tests) is green and ESLint is clean.