Function resolve_builtin_content 

pub fn resolve_builtin_content(
    id: &str,
    pane: &PaneState,
    is_active: bool,
    rs: &RenderState,
    provider_label: Option<&str>,
) -> ElementContent

Resolve a built-in (core.*) element’s content for pane. Pure reads off the published RenderState — O(1), no allocation proportional to document size (paramount #1).

is_active gates the elements that read active-document global state: core.mode (the modal label is a single active-doc value — showing it on an inactive pane would be wrong, not just noisy) and core.lang (parity with the legacy footer, which omitted it on inactive panes). provider_label overrides core.path for provider-backed panes (file-tree / oil / help).

An empty ElementContent means “hidden this frame” — the caller skips it (the same contract lattice_mode::ModelineSnapshot::zone uses).