Render: long roles wreck Mermaid edge labels, and ownership is dropped when a role is set

[jbeda]

Two related problems in relationshipLabel (internal/render/mermaid/mermaid.go), both
found while investigating #26 on a real 13-entity model.

1. Long role values collide as edge labels

The schema documents role as "What role the related entity plays ... A role name like
Owner should be defined in the glossary" — i.e. a short noun phrase. Nothing enforces
that. modelith lint only inspects backticked PascalCase tokens inside a role
(entityRefs), so a freeform prose role passes clean:

- entity: AcceptancePlan
  cardinality: "1:0..1"
  role: "the run's contract, present at capability scale"

The renderer passes it verbatim into the Mermaid edge label. Mermaid's dagre ER layout
reserves no space for edge labels, so on a model with several such roles the labels overlap
each other and the entity boxes, and drift onto lines they don't belong to. Replacing every
label with a single word cleared the overlap entirely — confirming the label text, not the
graph shape, is the cause.

Modelith has a better home for prose: relationship.note. But nothing steers an author
there, and the failure mode only shows up after rendering.

Options: lint warning when a role exceeds ~N words or contains sentence punctuation
(pointing at note); and/or wrap or truncate the label in the Mermaid renderer while
keeping the full text in the Markdown table.

2. Ownership is silently dropped when a role is present

func relationshipLabel(rel model.Relationship) string {
	switch {
	case rel.Role != "":
		return sanitize(rel.Role)
	case rel.Ownership != "":
		return rel.Ownership
	default:
		return rel.Cardinality
	}
}

Crow's-foot notation cannot express composition, so "owned" on the edge is the only place
that signal appears in the diagram. But the role branch wins, so:

• an owned relationship with a role renders the role, and looks identical to a
  referenced one;
• an owned relationship without a role renders "owned".

On the model I looked at, four relationships were ownership: owned; two showed owned and
two did not, purely because the latter also had a role. The diagram reads as though the
composition structure is inconsistent when the source is not.

Worth considering: combine them (owned — <role>), or drop the ownership fallback and
express composition through styling instead. Either way the current behaviour is information
loss that depends on an unrelated field being set.