JBZoo / Mermaid-PHP / 30002095152

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feat: clickable node links and per-link styles (#32)

## Summary

Adds two Mermaid flowchart features, both fully opt-in and backward
compatible:

1. **Clickable nodes** — attach a hyperlink to a node, rendered as a
Mermaid `click` directive with optional tooltip and target.
2. **Per-link styles** — give an individual link its own CSS, rendered
as `linkStyle <index> ...;` with the index resolved automatically.

Reimplements, on the current 8.x codebase, the features originally
proposed by @biscofil in #7 (which was merged for authorship; its 2021
code was incompatible with the rewritten codebase).

## API additions (all additive — no breaking changes)

**`Node`**
- Constructor gains a trailing `?string $url = null`.
- `setUrl()/getUrl()`, `setTooltip()/getTooltip()`,
`setTarget()/getTarget()`.
- Target constants: `Node::TARGET_BLANK`, `TARGET_SELF`,
`TARGET_PARENT`, `TARGET_TOP`.
- `getClickStatement(): ?string`.

**`Link`**
- Constructor gains a trailing `?string $css = null`.
- `setCss()/getCss()`.

**`Graph`**
- `addLinkByIds()` gains a trailing `?string $css = null`.
- `render()` now emits a global `click` / `linkStyle` section (only when
the features are used).

```php
$graph = (new Graph())
    ->addNode($a = (new Node('A', 'Docs'))->setUrl('https://github.com/JBZoo/Mermaid-PHP')->setTarget(Node::TARGET_BLANK))
    ->addNode($b = new Node('B'))
    ->addLink((new Link($a, $b))->setCss('stroke:blue,stroke-width:4px'));
// ...
// linkStyle 0 stroke:blue,stroke-width:4px;
// click A "https://github.com/JBZoo/Mermaid-PHP" _blank
```

## Backward compatibility

- `Link::setStyle(int)` and all existing constructor signatures,
constants and `__toString()` behaviour are untouched — new state is
added via new methods and trailing optional parameters only.
- A graph that uses no url/css renders byte-for-byte identically to
before; every pre-existing test assertion passes unmodified (they are
the behavioural contract), plus an explicit
`testBackwardCompatO... (continued)

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