Add public test helpers for rendering components

Testing a component means rendering it to a string and checking the result, and every project writes the same small helper to do that:

```go
func render(t *testing.T, n Node) string {
t.Helper()
var b strings.Builder
if err := n.Render(&b); err != nil {
t.Fatal(err)
}
return b.String()
}
```

gomponents has this internally in `internal/assert`, with `assert.Equal(t, expected, node)` comparing the rendered string, but it isn't importable.

Proposal: a small public package with helpers along these lines:

- render a node to a string, failing the test on a render error
- check that the rendered output equals an expected string
- check that the rendered output contains a substring, which is what most component tests want, since pinning a whole page as one exact string breaks on every unrelated change

The package name and exact API are open. It must keep the core library free of external dependencies, which `testing` and `strings` satisfy.

This came up while writing the coding agent skill in #358, where the skill currently has to tell agents to write the render helper themselves.

Start by reading the existing helpers in internal/assert and the related coding agent skill work in issue #358. The work is complete when a public package exposes helpers for rendering nodes, comparing exact output, and checking substrings without adding dependencies beyond testing and strings.

Written by the indexing model from the issue text.