Report malformed Object::Pad BUILD declarations clearly #1808

Context

PR #1804 added support for valid Object::Pad BUILD { ... } blocks, including execution after field initialization, and passed the CAD::OpenSCAD 0.14 acceptance tests on both JVM and interpreter backends. The original parser failure is fixed.

Remaining diagnostic gap

Malformed or misplaced BUILD syntax should receive a focused compile-time diagnostic. Today, an incomplete class block can fall through into ordinary expression parsing and fail later with a confusing runtime error.

For example, against the merged implementation:

use Object::Pad;
class BadBuild { BUILD method value {} }

currently reports Can't call method "value" on unblessed reference instead of identifying the malformed BUILD declaration.

Follow-up

• Detect BUILD in a class block when it is not followed by a block body and report a targeted syntax diagnostic.
• Add project-owned coverage for malformed and out-of-class BUILD declarations.
• Keep valid Object::Pad BUILD blocks and their constructor behavior unchanged.

Related: #1450, PR #1804.