007 registry field comprehension

// RED PIN (type-system design walk, 2026-07-06/07). A TYPE
// COMPREHENSION over the registry: a struct block row may be a
// comprehension clause whose DOMAIN is another registered type's
// fields — deriving a type by ranging over an entry. This is the
// registry acting as a comprehension domain (`f over Player`), the
// core unification of the walk: table comprehensions range over
// values, type comprehensions range over registry entries, one closed
// grammar. GUARD WORD deliberately absent — keep-vs-when is an open
// Lars ruling (O4 leans `when`); this pin uses no guard so it doesn't
// pre-empt it. Row spelling PROVISIONAL.

~import std/types
~import std/io

const std = @import("std");

~std/types:struct(Player) {
    name: []const u8,
    health: i64,
}

// Derive: every field of Player, plus one literal row.
~std/types:struct(PlayerSnapshot) {
    f over Player -> f
    tick: i64,
}

~event main {}

~proc main|zig {
    const s = PlayerSnapshot{ .name = "Alice", .health = 100, .tick = 7 };
    std.debug.print("{s} {d} @{d}\n", .{ s.name, s.health, s.tick });
}

~main()