031 store query sweep take

// A query branch can TAKE its matched row — "sweep the dead and remove them".
// Two things this pins:
//   1. Addressing: `take(arena[entity])` inside a query body addresses the
//      currently-swept row. `entity` there is the query's row cursor, rewritten
//      to the qbody cursor (not a stray identifier leaking as raw host code).
//   2. swap-remove during the sweep: take drops the LAST row into the freed
//      slot, so the sweep must re-check that index instead of advancing past
//      the row that just moved in. Row order [5, 50, 8] is the adversarial
//      case — taking row 0 (hp 5) swaps hp 8 into slot 0, already passed; the
//      sweep must revisit it. Both 5 and 8 are taken; 50 survives the guard.

import std/io
import std/store

std/store:new(arena) { hp: i64 }

std/store:insert(arena) { hp: 5 }
| row _ |> _
| full |> _
std/store:insert(arena) { hp: 50 }
| row _ |> _
| full |> _
std/store:insert(arena) { hp: 8 }
| row _ |> _
| full |> _

std/store:query(arena)
! query { entity.hp } when hp <= 10 |> std/store:take(arena[entity])
    | item i |> std/io:print.ln("took {{ i.hp:d }}")