BridgeJS: Diagnose struct initializer parameter order mismatch

[krodak]

Overview

Fixes #718. Adds a diagnostic when a @JS struct's explicit @JS init has parameters in a different order than the struct's stored property declarations.

The stack ABI pushes/pops struct fields in declaration order, so bridgeJSStackPop() always reconstructs the struct using property declaration order. If the user writes an init with a different parameter order, the generated code fails to compile with a confusing Swift error. This change catches the mismatch at codegen time with a clear message.

Example diagnostic:

test.swift:5:5: error: @JS struct initializer parameters must match stored properties in declaration order. Expected (size, age), got (age, size)
  5 |     @JS init(age: Int, size: Double) {
    |     `- error: ...

What changed:

• SwiftToSkeleton.visitPost(_: StructDeclSyntax) now validates that when an explicit @JS init exists, its parameter labels match the struct's property names in declaration order.
• If no @JS init is declared, the synthesized memberwise init is assumed (always correct).
• Three new tests: mismatched order produces diagnostic, matching order succeeds, no explicit init succeeds.