fix(core): silence three clippy warnings blocking CI gate

* line_index.rs: replace manual `x % BLOCK_SIZE == 0` with
  `.is_multiple_of(BLOCK_SIZE)` (Rust 1.81+).

* line_index.rs: gate three pub(crate) accessor methods
  (`sampled_offsets`, `total_lines`, `has_trailing_newline`) behind
  `#[cfg(test)]`. They are only consumed by tests in file_reader.rs;
  marking them test-only removes them from production builds entirely,
  eliminating the dead_code warning without suppressing it.

* json.rs: drop redundant `Some(... .ok()?)` wrapper in
  parse_json_object_with_duplicates. `.ok()` already returns Option,
  so wrapping it in Some and unwrapping with ? was a no-op.

No behavior change. Unblocks `cargo clippy --workspace -- -D warnings`.

crates/core/src/io/line_index.rs

@@ -158,7 +158,7 @@ impl LineIndex {
 
         // If the junction falls on a block boundary, record the start offset
         // (analogous to from_bytes always pushing offset 0 for line 0).
-        if starts_new_line && (old_total as usize) % BLOCK_SIZE == 0 {
+        if starts_new_line && (old_total as usize).is_multiple_of(BLOCK_SIZE) {
             self.sampled_offsets.push(start_offset);
         }
 
@@ -212,15 +212,17 @@ impl LineIndex {
         self.total_lines as usize
     }
 
-    // ─── getter 方法 ────────────────────────────────────────────────────
+    #[cfg(test)]
     pub(crate) fn sampled_offsets(&self) -> &[u64] {
         &self.sampled_offsets
     }
 
+    #[cfg(test)]
     pub(crate) fn total_lines(&self) -> u64 {
         self.total_lines
     }
 
+    #[cfg(test)]
     pub(crate) fn has_trailing_newline(&self) -> bool {
         self.has_trailing_newline
     }