fix(io): guard sampled_offsets index in get_line() to prevent panic on corrupt cache

Replace direct indexing with .get()? to return None instead of panicking
when sampled_offsets is shorter than expected (e.g. corrupt bincode cache).

Closes #3

crates/core/src/io/line_index.rs

@@ -233,7 +233,7 @@ impl LineIndex {
         }
         let block = idx / BLOCK_SIZE;
         let offset_in_block = idx % BLOCK_SIZE;
-        let mut pos = self.sampled_offsets[block] as usize;
+        let mut pos = (*self.sampled_offsets.get(block)?) as usize;
         for _ in 0..offset_in_block {
             match memchr::memchr(b'\n', &data[pos..]) {
                 Some(rel) => pos = pos + rel + 1,