feat: function pre/post conditions

[ganehag]

Problem

Gengo has predicates on types (checked at construction), but no way to express constraints at the function boundary. Ada's with Pre => and with Post => aspects allow contracts that the type alone cannot express.

func divide(a int, b int) int
    pre  b != 0
    post result == a div b
{
    return a div b
}

Use cases

• Preconditions on inputs that the type alone cannot enforce (e.g. "second argument must not be zero", "array must be non-empty")
• Postconditions for documentation and debugging (checked in debug builds, elided in release)
• Richer error messages than a runtime panic: "precondition violated: b must not be zero"

Design questions

• Checked always, or only in a debug/assert build mode?
• Can preconditions reference multiple parameters together (cross-parameter constraints)?
• Syntax: pre/post as keywords, or requires/ensures, or aspect-style annotations?
• Interaction with named return values: postconditions can reference named returns.
• Should violation be a panic (like assert) or a typed error?

Needs design before implementation.