syntax: replace impl blocks with named extension declarations

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Summary

Replace anonymous impl Foo { } blocks with named extension declarations (extension FooExtensions for Foo { }) that are first-class syntax nodes, have an explicit name, and can contain the same members as a class body except fields.

Motivation

The current impl blocks are semantically anonymous — multiple impl blocks for the same type are merged silently, and there is no way to refer to a specific extension as a unit. Named extensions make the grouping explicit, match how the type system already models them internally, and enable future features (conditional extensions, visibility on the extension itself, etc.).

Changes

Before:

impl Vector2 {
    def length(self): float { ... }
}

After:

extension Vector2Extensions for Vector2 {
    def length(self): float { ... }

    # Extensions also support associated let/const (no fields):
    let UNIT_X: Vector2 = Vector2(1.0, 0.0);
    const MAX_COMPONENTS: int = 2;
}

Extension members are accessible via the extended type (Vector2.UNIT_X, v.length()), not via the extension name.

Affected components

• TokenKind.java — add ExtensionKeyword; remove ImplKeyword
• SyntaxFacts.java — add ExtensionKeyword to keyword map
• ImplBlockSyntax.java — rename/replace with ExtensionDeclarationSyntax; add extensionName: SimpleIdentifierSyntax and targetType: TypeSyntax fields; keep members: SyntaxList<MemberSyntax>
• Parser.java — replace parseImplBlock() with parseExtensionDecl(): consume extension, parse name identifier, consume for, parse target type, parse member block; update isMemberStart() and parseMember() to use ExtensionKeyword
• SyntaxVisitor.java, SyntaxWalker.java — rename visitImplDecl → visitExtensionDecl
• MemberBinder.java / GlobalBinder.java — update all call sites that handle ImplBlockSyntax
• TestParser.java, TestTypeChecker.java — update all impl fixtures

Notes

• for in extension X for Y does not conflict with the loop ForKeyword because the parser only enters parseExtensionDecl() after matching ExtensionKeyword. Inside that production for is consumed as a separator token rather than a statement keyword.
• Extensions cannot declare fields; attempting to do so is a semantic error (reported by the binder, not the parser, to keep the grammar simple).
• The extension name (FooExtensions) is currently unused at runtime but is preserved in the AST for tooling and future use.