185 param default on subflow path

// PIN (aspirational) — an input DEFAULT holds on the inlined subflow path.
//
// `b: i32 = 5` works today only on the proc path (400_186) and only by
// accident: the default was never parsed as a concept. It rides along inside
// the field's TYPE STRING — `field.type` is literally `"i32 = 5"` — which the
// emitter pastes verbatim into the generated Zig `Input` struct, where ZIG
// applies it. A bare-return subflow has no Input struct: args bind straight
// from the call site, the type string is never used, and the default
// evaporates into `use of undeclared identifier 'b'`.
//
// `src/auto_discharge_inserter.zig` carries the honest note next to its
// auto-fill classifier: `TODO: event-input defaults when the surface supports
// them on shapes`. It does not support them; it leaks them.
~import std/io

~tor add { a: i32, b: i32 = 5 } -> i32

~add -> a + b

~add(a: 1): r |> std/io:print.ln("{{ r:d }}")