Use string literals for alignX/alignY instead of magic numbers

[cowboyd]

Currently alignX and alignY take raw numbers that map to Clay's internal enum:

// alignX: LEFT=0, RIGHT=1, CENTER=2
// alignY: TOP=0, BOTTOM=1, CENTER=2
alignX: 2, alignY: 1  // what does this mean?

Change the type from number to string literal unions:

alignX?: "left" | "center" | "right";
alignY?: "top" | "center" | "bottom";

The conversion to 0/1/2 happens in pack(). Callers get readable code with zero helpers:

alignX: "center", alignY: "bottom"

V8 interns these short strings so there's no performance cost vs integers.

Files to change

• ops.ts — update OpenElement interface and pack() function
• validate.ts — update schema from u8 to string enum