Function project_decoration_context 

pub fn project_decoration_context(
    buffer_id: u64,
    path: Option<&Path>,
    line_count: u32,
) -> DecorationContext

Build the owned decoration-context the host hands a producer (host→guest, one-way). The host has the buffer metadata off the render path when it triggers the producer; the guest computes per-line decorations from these scalars (+ host-services for external data like git HEAD). A non-UTF-8 path is dropped to None (a decoration producer keys off the buffer, not the path text — losing an un-representable path degrades gracefully rather than failing the whole trigger, unlike an event delivery).