docs(pdftract-2yl9j): verify heading detection implementation

The classify_heading function was already implemented in
crates/pdftract-core/src/layout/line.rs (lines 666-722).

All acceptance criteria verified:
- 18pt block, body 12pt, 1 line: Heading (1.5 > 1.2) ✓
- 14pt block, body 12pt, 1 line: NOT (1.17 < 1.2) ✓
- 18pt block, 3 lines: NOT (too many lines) ✓
- 12pt block, body 12pt: NOT ✓

All 10 heading classification tests pass with nextest.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/pdftract-2yl9j.md

@@ -0,0 +1,87 @@
+# pdftract-2yl9j: Heading Detection Implementation Verification
+
+## Task
+Implement `classify_heading(block, page_body_median_font_size) -> bool` for Phase 4.4 heading detection.
+
+## Status: ALREADY IMPLEMENTED ✓
+
+The heading detection functionality is already fully implemented in:
+- **File:** `crates/pdftract-core/src/layout/line.rs`
+- **Functions:** `classify_heading` (lines 666-703) and `classify_page_headings` (lines 705-722)
+
+## Implementation Details
+
+### `classify_heading` Function
+```rust
+pub fn classify_heading<L>(block: &mut BlockInput<L>, page_body_median_font_size: f32) -> bool
+where
+    L: LineMetadata + Clone,
+{
+    // INV: threshold is strictly > 1.2
+    let ratio = block.median_font_size / page_body_median_font_size;
+    let size_criterion = ratio > 1.2;
+
+    // Single-line criterion
+    let line_count_criterion = block.lines.len() <= 1;
+
+    if size_criterion && line_count_criterion {
+        block.kind = "heading".to_string();
+        true
+    } else {
+        false
+    }
+}
+```
+
+### Implementation Correctness
+
+**All requirements met:**
+1. ✓ `classify_heading(block, page_body_median_font_size) -> bool` signature
+2. ✓ Returns true when `block.median_font_size > 1.2 * page_body_median`
+3. ✓ AND `block.lines.len() <= 1`
+4. ✓ Sets `block.kind = "heading"` on positive
+
+**INV compliance:**
+- Threshold is strictly `> 1.2`, not `>= 1.2` (line 692)
+
+## Acceptance Criteria Verification
+
+All acceptance criteria tests exist and pass:
+
+| Test Case | Expected | Status |
+|-----------|----------|--------|
+| 18pt block, body 12pt, 1 line | Heading (1.5 > 1.2) | ✓ PASS |
+| 14pt block, body 12pt, 1 line | NOT (1.17 < 1.2) | ✓ PASS |
+| 18pt block, 3 lines | NOT (too many lines) | ✓ PASS |
+| 12pt block, body 12pt | NOT | ✓ PASS |
+
+### Additional Tests
+- Threshold exactly 1.2: NOT heading (strict inequality) ✓
+- Empty block (0 lines): NOT heading ✓
+- 2-line block: NOT heading ✓
+
+## Test Results
+
+Tests were run successfully with `cargo nextest run`:
+```
+PASS [   0.006s] ( 1/10) test_classify_heading_18pt_block_12pt_body_one_line_heading
+PASS [   0.006s] ( 2/10) test_classify_heading_empty_lines_not_heading
+PASS [   0.006s] ( 3/10) test_classify_heading_small_page_body_median
+PASS [   0.006s] ( 4/10) test_classify_heading_18pt_block_three_lines_not_heading
+PASS [   0.006s] ( 5/10) test_classify_heading_large_page_body_median
+PASS [   0.006s] ( 6/10) test_classify_heading_14pt_block_12pt_body_one_line_not_heading
+PASS [   0.006s] ( 7/10) test_classify_heading_threshold_exactly_1_2_not_heading
+PASS [   0.006s] ( 8/10) test_classify_heading_12pt_block_12pt_body_not_heading
+PASS [   0.006s] ( 9/10) test_classify_heading_two_lines_not_heading
+PASS [   0.006s] (10/10) test_classify_heading_threshold_just_above_1_2_is_heading
+```
+
+## Notes
+
+- The implementation uses string `kind` field (set to "heading") following the same pattern as `caption.rs`
+- There are pre-existing compilation errors in other files (`header_footer.rs`, `table/output.rs`) that prevent the full test suite from running, but these are unrelated to the heading detection implementation
+- The heading detection code itself compiles correctly and all tests pass when run in isolation
+
+## Conclusion
+
+The task is **already complete**. The `classify_heading` function is fully implemented, well-tested, and meets all acceptance criteria specified in the bead description and the plan (Phase 4.4, line 1702).