refactor(vector): address PR #775 review round 3 — fix unused var, harden boundary lookup, rename trace span

- pdf_highlighter.compute_chunk_bboxes_batch: drop unused chunk_text
  destructure (SonarQube finding), and replace positional
  page_boundaries[page_num - 1] with a key-based next() match so
  reordered or non-1-indexed boundaries can't silently shift the bbox.
  Convert touched f-string log to lazy %s formatting.
- vector/processor: rename the trace_operation span from
  "vector_sync.generate_highlights" to "vector_sync.compute_chunk_bboxes"
  to match what the function actually does.
- Add test_compute_chunk_bboxes_handles_unordered_page_boundaries —
  reverses the boundaries list and asserts identical results to the
  in-order case, guarding the boundary-lookup regression class.
- Pin pre-push-review skill to sonnet model.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

.claude/skills/pre-push-review/SKILL.md

@@ -7,6 +7,7 @@ description: |
   in this repo's automated PR reviews. Use when the user is about to push, says "ready
   to push", "review my work", "check before PR", or invokes /pre-push-review.
   Report-only — does not modify code.
+model: sonnet
 allowed-tools:
   - Bash
   - Read