Add oam_begin() and OamFrame ref struct to neslib

[Copilot]

Phase 2 of using var pattern support (#427). Adds the neslib API surface for RAII-style OAM sprite drawing scopes.

• OamFrame ref struct — ensures compiler emits direct call to Dispose() instead of callvirt through IDisposable (no boxing, no interface dispatch, simpler IL for transpiler)
• oam_begin() — returns OamFrame; stub follows existing throw null! pattern
• PublicAPI.Unshipped.txt updated with new surface

using var frame = oam_begin();
oam_meta_spr_pal(x, y, pal, sprite);
// Dispose() auto-calls oam_hide_rest(oam_off) at end of scope