refactor: Replace gettype() checks with union type declarations

[usernane]

Description

The codebase uses gettype() string comparisons for type branching — a PHP 5-era pattern. Since the library requires PHP 8.1+, these should be union type declarations on parameters, letting PHP enforce types at call time.

Examples

// Before
public function addChild($node, $attrsOrChain = [], bool $chainOnParent = false) {
    if (gettype($node) == 'string') {
        $toAdd = new HTMLNode($node);
    } else {
        $toAdd = $node;
    }
    $sType = gettype($attrsOrChain);
    if ($sType == 'boolean') { ... }
}

// After
public function addChild(HTMLNode|string $node, array|bool $attrsOrChain = [], bool $chainOnParent = false): HTMLNode {
    $toAdd = is_string($node) ? new HTMLNode($node) : $node;
    $chain = is_bool($attrsOrChain) ? $attrsOrChain : $chainOnParent;
}

Scope

Methods that need updating (non-exhaustive):

• HTMLNode::addChild() — $node, $attrsOrChain
• HTMLNode::setAttribute() — $val
• HTMLNode::setAttributes() — $attr key type check
• HTMLNode::build() — $child type check
• HTMLNode::component() — $loaded return check
• HTMLNode::removeChild() — $nodeInstOrId
• HTMLList::addChild() — $node
• HeadNode::addChild() — $node
• TableRow::addChild() — $node
• TemplateCompiler::setComponentVars() — $varsArr
• TemplateCompiler::setSlotsHelper() — $slotVal

Benefits

• Static analysis tools (PHPStan/Psalm) can verify call sites
• TypeError thrown immediately on invalid input instead of silent no-op
• Clearer API contracts in IDE autocomplete
• Removes ~40 gettype() calls

Backward Compatibility

Fully compatible for callers passing correct types. Callers passing null or wrong types that were silently ignored will now get TypeError — this surfaces existing bugs in caller code.

Acceptance Criteria

• Zero gettype() calls remain in source code (replace with is_string(), is_array(), is_bool() where branching is still needed)
• All public method parameters have declared types
• All 308 tests pass unchanged