MDS051 heading ready

MDS051: single-h1

At most one H1 heading is allowed per file.

A second H1 usually indicates two documents merged into one or a heading-level mistake.

# Settings

Setting	Type	Default	Description
`front-matter-title`	string	`"title"`	Front-matter field that counts as an H1. When the field is set and the file also contains an H1, the H1 is flagged. Set to `""` to disable.

Enable:

rules:
  single-h1: true

Disable (default):

rules:
  single-h1: false

Ignore front-matter title:

rules:
  single-h1:
    front-matter-title: ""

# First

## Section

# Second

---
title: My Doc
---

# My Doc

# Title

## Section

### Sub-section

Message	Condition
`extra H1 heading; only one H1 is allowed per file`	A second or later H1 heading exists in the document
`h1 heading conflicts with front-matter title`	An H1 heading exists and the front matter contains the configured field with a non-empty string value

ID: MDS051
Name: single-h1
Status: ready
Default: disabled (opt-in)
Fixable: yes — extra H1s are demoted to H2; front-matter conflicts are not auto-fixed
Implementation: source
Category: heading
markdownlint: MD025 (single-h1)
rumdl: MD025 (single-title)
mado: MD025 (single-h1)
gomarklint: single-h1