004 store no torn state

// ASPIRATIONAL (RED by design until std/store rung 1 lands — see DESIGN.md).
// NO TORN STATE — the headline guarantee ((h) lean, Lars-ruled direction):
// the atomicity unit is a write PLUS its full interceptor cascade; writes
// interleave, never overlap; no watch ever observes mid-cascade state.
//
// Here the interceptor derives shield from hp (same-store other-field
// write: an ACYCLIC field-level edge hp→shield — legal under the
// field-granular cycle graph, DESIGN.md queue (d)). The interceptor
// discards its payload (`! hp _` — corpus discard-token shape; a bare
// unbound arm is only legal for void payloads, 400_096). ASSUMES the
// creation seed does NOT fire watches (birth-is-not-a-write — DESIGN.md
// O9, lean pending ratification). The watch on hp reads
// game.shield in its body (comptime-known store name — transplant-pure).
// The pin: by the time ANY watch fires, the cascade has settled, so hp
// and shield are always observed consistent (shield == hp * 2). A torn
// implementation would print "hp 5 shield 0".

~import std/io
~import std/store

~std/store:create(game) { hp: 0[i64], shield: 0[i64] }
! hp _ |> std/store:stored { game.shield: game.hp * 2 }

~std/store:watch(game)
! hp h |> std/io:print.ln("hp {{ h:d }} shield {{ game.shield:d }}")

~std/store:stored { game.hp: 5 }
~std/store:stored { game.hp: 9 }