079 effect branch struct payload

// Test: effect branch with struct payload (multi-field).
//
// Producer fires `token(.{ .kind = N, .lexeme = "..." })` with a struct
// payload. Consumer's `! token t |> ...` binds `t` and accesses `t.kind`
// and `t.lexeme`. Proves the Handlers struct fn signature emits an anon
// struct param for multi-field payloads, replacing the `@compileError`
// placeholder from Phase 3b first cut.

~import std/io

~pub event tokenize { source: []const u8 }
! token { kind: u32, lexeme: []const u8 }

~proc tokenize|zig {
    _ = source;
    token(.{ .kind = 1, .lexeme = "first" });
    token(.{ .kind = 2, .lexeme = "second" });
}

~tokenize(source: "x")
! token t |> std/io:print.blk {
    token kind={{ t.kind:d }} lex={{ t.lexeme:s }}
}