feat(apidoc): predictable heading hierarchy enforcement (h2/h3/h4 by symbol depth)

[webmaven]

Overview

API reference pages must emit headings at fixed absolute HTML levels
regardless of where golem:apidoc[] appears in a document.
Currently injected headings shift relative to nesting depth, breaking
screen reader navigation and WCAG 2.1 SC 1.3.1.

Required Mapping

HTML level	Meaning
<h1>	Module title
<h2>	Top-level exported symbol (Class, Function, Exception)
<h3>	Method, property, class variable
<h4>	Parameter category, nested type attribute

Implementation Approach

The apidoc Chameleon templates (in the plugin's own template directory per
webmaven/asciidoctype#7) render headings as literal <h2>/<h3>/<h4>
HTML, bypassing AsciiDoctrine's heading-level normaliser entirely.

A cleaner long-term alternative is webmaven/asciidoctrine#128
(absolute-level ASG node attribute). If that lands first, the
NodeTransformer in #10 can set absolute-level on injected headings instead.

Dependencies

Requires:

• feat(apidoc): AST-level NodeTransformer macro splicing for golem:apidoc[] #10 — AST-level NodeTransformer
• refactor(engine): transition from raw dict ASG manipulation to typed ASG domain models #14 — typed ASG
• feat: plugin-local template directory registration in AsciiDoctypeRenderer asciidoctype#7 — plugin-local template dir

Enabled by (makes this cleaner):

• feat: absolute-level attribute on heading ASG nodes to override document-relative depth asciidoctrine#128 — absolute-level heading attribute

Acceptance Criteria

• Class/function/exception headings always <h2>.
• Method/property headings always <h3>.
• Parameter category headings always <h4>.
• Levels correct regardless of golem:apidoc[] placement depth.
• WCAG 2.1 SC 1.3.1 satisfied.