075 identity struct mismatch

// Test: Shape checker catches identity-vs-struct mismatch in subflow override.
//
// `greet` declares `| ok []const u8` (identity branch).
// The subflow override `~greet = ok { message: "Hello" }` uses struct-init syntax,
// which is only valid for multi-field branches. Single-field struct branches
// are illegal at declaration time (post commit 869cd29), so this constructor
// cannot possibly match any valid declaration of `ok`.
//
// Expected: shape checker at Stage C (analysis/check_structure) rejects with KORU030
// and does NOT let the malformed constructor reach Zig emission.

~event greet {}
| ok []const u8

~greet = ok { message: "Hello" }

~greet()
| ok _ |> _