[Parsing] Same-row headings above side-by-side tables are merged

[Chengyunlai]

Description

Two independent headings placed on the same y coordinate above two
side-by-side ruled tables can be emitted as one wrapped Markdown heading.

This is the remaining downstream boundary after the geometry and projected
table-line fixes in #392. At the tested head of #392, the two tables are
correctly independent, but their headings are still merged.

Document

• GitHub preview / download
• Raw PDF
• Fixture notes
• SHA-256: 8872a0f62295b9116504a1125c658e3f489290ce815981a66a958428c09cf22a

The PDF is self-authored and synthetic. It contains no customer, personal, or
project data. Each table has a nearby heading and exactly three rows by two
columns.

Expected output

## Release schedule

| Channel | Status |
|---|---|
| Stable | Ready |
| Preview | Testing |

## Support hours

| Region | Window |
|---|---|
| East | Morning |
| West | Afternoon |

Actual output

At Chengyunlai/liteparse@26b79a6, the tables are separate but the headings
are merged:

## Release schedule Support hours

| Channel | Status |
|---|---|
| Stable | Ready |
| Preview | Testing |

| Region | Window |
|---|---|
| East | Morning |
| West | Afternoon |

Command used

lit parse side-by-side-three-row-tables.pdf \
  --no-ocr \
  --format markdown \
  --output output.md \
  --quiet

A/B evidence

The same PDF and public command were run in the same Linux container:

Version	Grid geometry	Markdown tables	Headings
main@2fd644a	one merged component	one merged table	merged
PR #392 before projection fix, 9d740a4	two components	ruled candidates rejected	merged
PR #392 with projection fix, 26b79a6	two components	two independent tables	still merged

This shows that table-confidence thresholds and fallback are no longer the
remaining problem at 26b79a6.

Root-cause evidence

The projection stage already emits two distinct same-y lines:

[md] ... text="Release schedule"
[MD heading-emit size/outline] h2 ... 'Release schedule'
[md] ... text="Support hours"

Markdown classification then treats Support hours as a wrapped continuation
of Release schedule. continues_heading sees the same font size, bold style,
and region path, while the equal y coordinate produces a negative vertical
gap. It currently has no horizontal or independent-table ownership guard.

Scope and regression constraints

The fix should preserve legitimate wrapped multi-line headings. It should not:

• globally disable heading continuation;
• change ruled-table confidence thresholds;
• change fallback table detection.

Suggested regression coverage:

• same-y headings owned by separate side-by-side tables remain separate;
• a legitimate two-line wrapped heading still merges;
• one page-spanning heading above both tables remains one heading;
• ordinary non-table headings keep current behavior.

Environment

• LiteParse: current main@2fd644a and PR fix: keep side-by-side ruled tables separate #392 through 26b79a6
• Operating system: Linux, aarch64 Docker container
• OCR: disabled

Related: #392, #414