Function oneshot_highlight_lines 

pub fn oneshot_highlight_lines(
    lang: Lang,
    registry: Arc<LangRegistry>,
    source: &str,
    start_line: u32,
    end_line: u32,
) -> Option<Vec<Vec<StyledSpan>>>

Run a fresh tree-sitter parse over source and return per-line styled spans for lines in start_line..end_line.

Returns None when:

• lang is Lang::Plain (no grammar to apply),
• the registry has no registered grammar for lang,
• the parse or highlight call fails for any reason (registry lookup error, malformed source, etc.) — fail silently so the caller’s “no syntax” fallback path (monochrome rendering) takes over without a panic.

start_line and end_line are inclusive-exclusive in line indices, matching crate::SyntaxSnapshot::highlight_lines. Returns one Vec<StyledSpan> per line in the range. Spans within each inner vec are line-relative byte offsets.

Cost model. Tree-sitter parse is O(source_len); highlight query walk is O(matches). For the diff deletion-block case where a typical baseline is hundreds to thousands of lines, both are well under 10 ms on modern hardware — within DiffOverlayRefreshTask’s off-thread budget. Caller is expected to invoke at hunk-publish frequency (debounced at the session level), not per-frame.