Remove the javax.swing dependency from doxia-sink-api and doxia-core

### New feature, improvement proposal

`doxia-sink-api` and `doxia-core` pull in `java.desktop` for four types from
`javax.swing.text`. None of them is used for anything Swing does. All four serve as
constant tables or as a `String`-to-`Object` map.

| Type | Main sources | What Doxia uses it for |
| --- | --- | --- |
| `javax.swing.text.html.HTML.Attribute` | 6 | A table of attribute-name strings |
| `javax.swing.text.html.HTML.Tag` | 6 | A table of tag-name strings, plus `isBlock()` |
| `javax.swing.text.MutableAttributeSet` | 8 | The supertype of `SinkEventAttributes` |
| `javax.swing.text.AttributeSet` | 4 | Parameter and return types around the above |

#### `HTML.Attribute` is a string table

38 references in main sources; 33 are `Attribute.X.toString()`. The other five pass the
constant itself as the key: `XdocSink:389` and `Xhtml5Sink:169,174,190,191`. Those are
harmless only because `SinkEventAttributeSet.addAttribute` calls `name.toString()` on the
key, while `getAttribute`, `isDefined` and `removeAttribute` do not — so a set written
with one of those keys can only be read back with a `String`. `SinkEventAttributes`
already declares `CLASS`, `ID`, `HREF`, `SRC`, `STYLE`, `NAME`, `BORDER`, `LANG`, `VALUE`
and `REL` as `String` constants; of the names in use only `CONTENT` is missing.

#### `HTML.Tag` is a name holder whose one behaviour is already wrong here

`HtmlMarkup` declares 117 tags. 56 of them are already anonymous subclasses that override
nothing but `toString()`, because HTML5 has tags Swing's HTML 4 table never had. The only
inherited behaviour any Doxia code reads is `isBlock()`, at `AbstractXmlSink:113`, which
decides whether to write a newline before a start tag.

That answer is arbitrary for this codebase today. Every one of the 56 hand-rolled tags
inherits `isBlock() == false`, so `section`, `article`, `figure`, `main`, `nav`, `aside`,
`header`, `footer`, `thead` and `tbody` are all treated as inline. From the other
direction Swing reports `address`, `caption`, `form`, `hr` and `html` as non-block, and
`menu` and `title` as block.

#### `AttributeSet` carries machinery nothing uses

`SinkEventAttributes extends MutableAttributeSet`, and the implementation is a
`LinkedHashMap` plus a `resolveParent`. No production code in Doxia calls
`setResolveParent`; only `SinkEventAttributeSetTest` does. #1075 is a consequence of that
otherwise unused machinery. `entrySet()`, added in 2.1.0, is already a Doxia-owned
replacement for the `Enumeration getAttributeNames()` iteration.

### Why bother

Worth being plain about the size of the win. On a full JDK this costs nothing at runtime,
and Doxia never touches `HTMLEditorKit`, so no AWT is initialised. What it buys is a
smaller jlink image and jdeps graph, one less obstacle for GraalVM native-image, a future
`module-info` that need not `requires java.desktop`, and an attribute API with `String`
keys instead of `Object` ones.

The blast radius outside Doxia is small. Across a checkout of the Maven repositories only
three main sources name a Swing type at all: `DependenciesRenderer` in
maven-project-info-reports-plugin and `CpdReportRenderer` in maven-pmd-plugin, both only
`Attribute.X.toString()`, and `GeneratorUtils` in maven-plugin-tools-generators, which
uses `HTMLEditorKit` for its own HTML parsing and has nothing to do with the Sink API.

### Plan

**1. Drop `HTML.Attribute`. No API change, can go into 2.x.** Done in #1095.

Replace every `Attribute.X.toString()` with the `SinkEventAttributes` constant, adding
`CONTENT`. Fix the five call sites that pass an `Attribute` as a key. This removes one of
the four types on its own.

**2. Stop iterating attribute sets through Swing, as far as the types allow. No API change, 2.x.**
Done in #1095.

Only code that already holds a `SinkEventAttributes` can move to the `entrySet()` added in 2.1.0:
`Xhtml5BaseSink.escapeAttributeValues` and `SinkEventAttributeSet.toString()`.

This step originally also listed `SinkUtils.getAttributeString`, `asCssString` and
`filterAttributes`. That was wrong. All three take `javax.swing.text.AttributeSet`, which has no
`entrySet()`, and widening those parameters to `SinkEventAttributes` is a binary break. The same
applies to `Xhtml5BaseSink.writeStartTag(Tag, MutableAttributeSet, boolean)` and to the
`SinkEventAttributeSet` methods that implement the Swing interface. All of them wait for step 4.

**3. Introduce a Doxia tag type, additively, in 2.x.**

Add `org.apache.maven.doxia.markup.HtmlTag` carrying a name and an explicit `isBlock`,
with parallel `writeStartTag(HtmlTag, ...)` overloads on `AbstractXmlSink` and the
`Tag`-typed ones deprecated. Copy the current `isBlock` values verbatim so no generated
site's whitespace moves; correcting the block table is a separate, deliberate change with
its own before-and-after site diff.

**4. Cut the Swing supertype, next major.**

`SinkEventAttributes` declares its own `String`-keyed methods and stops extending
`MutableAttributeSet`; the `HtmlMarkup` constants retype to `HtmlTag`; the public
`AttributeSet` parameters on `SinkUtils` retype to `SinkEventAttributes`; the deprecated
overloads go. A one-method adapter can hand a `javax.swing.text.AttributeSet` view to
anyone who genuinely wanted one.

### Compatibility

Steps 1 and 2 are internal. Step 3 is additive.

Step 4 is why this is a major-version item, on two counts. Retyping the `HtmlMarkup`
constants changes their field descriptors, which no amount of deprecation can stage. And
dropping a superinterface is a japicmp break by definition, although a mild one in
practice: if `SinkEventAttributes` re-declares the same method names, an existing
`invokeinterface SinkEventAttributes.getAttribute` still resolves, and the only code that
actually breaks is code that hands a `SinkEventAttributes` to a parameter typed
`AttributeSet`. The survey above found no such call site in the Maven repositories.

### Related

#1072 / #1073 moved `SinkEventAttributeSet` into `doxia-sink-api`, which puts the
interface and its only implementation in the same module — where steps 2 and 4 have to
happen. #1074 retypes the protected parser hooks to `SinkEventAttributes`, which is the
same major-version window as step 4. #1075 is the `resolveParent` symptom.

*This issue was created with AI assistance.*

Start with the plan and inspect HtmlMarkup, AbstractXmlSink, SinkEventAttributes, SinkEventAttributeSet, and SinkUtils, along with the current tests and the completed work described for #1095. Trace the remaining javax.swing.text types and compatibility-sensitive signatures; done means the planned HtmlTag and attribute API changes are implemented with the required overload and API checks, without unintended generated-site whitespace changes.

Written by the indexing model from the issue text.