332 store write inside custom effect arm

// PINS (aspirational): a std/store write inside a USER-DEFINED `!`-pulse arm
// is a real site — the same accumulation shape the suite already proves inside
// std-provided pulses (`std/grid:sweep`'s `! sweep` arm in 2105_prime_sieve).
//
// Today the store transform's site fold does not see sites inside custom
// effect-branch arms: `std/store:stored` here emits a `SL.ColProv` reference
// the synthesized SL struct lacks, plus an `ast_find.__koru_ast` path that
// does not exist — BACKEND_COMPILE_ERROR instead of a working write. Same
// transform-blindness family as 400_098 (capture over a custom pulse).
//
// Expected once fixed: evens pulses 2,4,6,8,10; the arm accumulates into the
// store; `| done` reads `acc.sum` = 30.

~import std/io
~import std/store

const std = @import("std");

std/store:new(acc) { sum: 0[i64] }

~pub tor evens { n: usize }
! item i64
| done i64

~proc evens|zig {
    var i: usize = 2;
    while (i <= n) : (i += 2) {
        item(@as(i64, @intCast(i)));
    }
    return .{ .done = @intCast(n / 2) };
}

~evens(n: 10)
! item v |> std/store:stored { acc.sum: acc.sum + v }
| done _ |> std/io:print.ln("sum {{ acc.sum:d }}")