006 store reject watch body ambient context

// NEGATIVE TEST (designed rejection — transplant purity, DESIGN.md T1 +
// ruling (a)). A watch body executes at the store's WRITE SITES, not at
// its lexical position — so it may reference only its own branch bindings
// and comptime-known names (module decls, qualified calls, store names).
// Here the body reaches for `ctx`, a RUNTIME binding of the enclosing
// flow: rejected, with a koru-level diagnostic (never a raw Zig
// undeclared-identifier leak — the check lives IN the store transform,
// rewrite-time, per the 2026-07-04 ruling).
//
// The watch's lexical placement mid-flow is otherwise LEGAL (links live
// anywhere — organizational principle, ruling 4); placement is not the
// crime, the ambient reference is.
//
// Branch-payload grammar: single-payload branches use identity syntax
// (`| ready []const u8` — the parser rejects single-field braces,
// PARSE003); whole-payload binding on a `|` branch (`| ready ctx |>`)
// grounded via 910_LANGUAGE_SHOOTOUT/2104_17 (`| connected c |>
// app/db:tx.begin(conn: c)`). Note: expected_error.txt here is
// DOCUMENTARY (the designed diagnostic text) — the harness checks the
// EXPECT CONTAINS lines, not this file, when both are present.

~import std/io
~import std/store

~std/store:create(game) { entities: 0[i64] }

~pub event setup {}
| ready []const u8

~proc setup|zig {
    return .{ .ready = "local" };
}

~setup()
| ready ctx |> std/store:watch(game)
    ! entities e |> std/io:print.ln("{{ ctx:s }}: {{ e:d }}")