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[kotlin] Add XPath functions and type attributes (#6893)

Adds XPath function and attribute support to pmd-kotlin, enabling type-aware XPath rules.

## What's included

**7 XPath functions** (`pmd-kotlin:` namespace):
- `typeIs(typeName)` / `typeIsExactly(typeName)` — type checks (with/without subtypes)
- `matchesSig(signature)` — match function/constructor call signatures
- `hasAnnotation(fqn)` — check annotation presence
- `modifiers()` — get modifier keywords
- `isNullable()` — nullable type check
- `hasUnresolvedReference()` — whether a node has an unresolved type reference

**Type-info XPath attributes** (need `auxClasspath` + resolved types; absent otherwise):

| Attribute | Nodes | Meaning |
|---|---|---|
| `@TypeName` | ClassDeclaration, PropertyDeclaration, ClassParameter, DelegationSpecifier, CatchBlock, ForStatement, SingleAnnotation, UnescapedAnnotation, FunctionValueParameter | FQ type name (with generics + nullable marker) |
| `@ReturnTypeName` | FunctionDeclaration | FQ return type name |
| `@AnnotationFqNames` | ClassDeclaration, FunctionDeclaration, PropertyDeclaration, ClassParameter | FQNs of all annotations (always present, empty list if none) |

**General (structural) XPath attributes** — always present, don't depend on type resolution:

| Attribute | Nodes | Meaning |
|---|---|---|
| `@Identifier` | ClassDeclaration, FunctionDeclaration, ClassParameter, CompanionObject, VariableDeclaration, ImportAlias | Simple name of the declared identifier |
| `@Mutable` | PropertyDeclaration | `true`/`false` for `var`/`val` |
| `@Name` | ImportHeader | Fully-qualified imported name |

Note: modifiers are **not** exposed as an `@Modifiers` XPath attribute — use `pmd-kotlin:modifiers()`
instead. `HasModifiers.getModifiers()` remains available for Java-based rules.

**`KotlinTypeName` type abstraction.**
Type info is stored as `KotlinTypeName` objects (PMD-owned, immutable), decoupling the public API
from kotlin-type-mapper, which stays an inter... (continued)

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