003 store interceptor cross store sync

// ASPIRATIONAL (RED by design until std/store rung 1 lands — see DESIGN.md).
// INTERCEPTORS: effect branches on `create` are the store's CONTRACT —
// schema-level, unconditional, fire for every write from anywhere
// (DESIGN.md ruling 4). Here an interceptor keeps a second store coherent:
// cross-store synchronization, the canonical interceptor use (Lars, on the
// walk: "interceptors keep the store state coherent").
//
// The `updated { old, new }` payload is the (c)-lean write contract:
// old/new images are USAGE-SYNTHESIZED — the transform sees this
// destructure binds both, so the write path materializes the pre-image
// (read-modify-write), inside the write's atomic step. A program with no
// old-binding anywhere pays no read. Destructure grammar grounded:
// `! pair { a, b } |>` (020_018).
//
// hp goes 0→5 (delta +5), 5→12 (delta +7); total accumulates the deltas.
// ASSUMES the creation seed does NOT fire watches or interceptors
// (birth-is-not-a-write — DESIGN.md O9, lean pending ratification):
// no "total: 0" line.

~import std/io
~import std/store

~std/store:create(global) { total: 0[i64] }

~std/store:create(game) { hp: 0[i64] }
! updated { old, new } |> std/store:stored { global.total: global.total + new - old }

~std/store:watch(global)
! total t |> std/io:print.ln("total: {{ t:d }}")

~std/store:stored { game.hp: 5 }
~std/store:stored { game.hp: 12 }