[FEATURE] Support selecting an alternate template per document

[linawolf]

Add the generic :template: field-list metadata: TemplateNode,
TemplateFieldListItemRule to parse it, TemplateMetadataNodeRenderer to
render it as nothing (it's a marker, not visible content), and
DocumentNode::getTemplate() to expose the selected template name.

Split out of #1358 per review feedback -- the genindex work on that
branch is the first consumer (:template: genindex), but the
mechanism itself is generic and reviewable on its own.

Signed-off-by: lina.wolf lwolf@w-commerce.de

Assisted by claude-code:claude-opus-5 — Session

[jaapio]

Could you also include an integration test for this new feature?

[linawolf]

No integration test here: at this point :template: only reaches DocumentNode::getTemplate() — nothing consumes it yet, and TemplateMetadataNodeRenderer always renders it as an empty string regardless of the value. There's no observable output difference to assert end-to-end, so an integration test isn't possible for this PR in isolation.

The unit tests cover what's actually testable at this layer: TemplateFieldListItemRuleTest (parses :template: case-insensitively into a TemplateNode) and TemplateMetadataNodeRendererTest (only TemplateNode renders as nothing) — same pattern as the other marker fields (orphan, navigation-title).

An integration/fixture test showing the template actually affecting output belongs with the consumer — the genindex work on the follow-up branch, which reads getTemplate() === 'genindex'.