`build_block_convert(args: []Type, $ret: Type) -> string` emits
the convert-body source for the generic `Into(Block) for
Closure(..$args) -> $R` impl (step 5.2):
1. A nested `__invoke :: (block_self: *Block, arg0: T0, ...) ->
R callconv(.c) { ... }` trampoline matching the per-shape
Apple Block ABI.
2. A `return Block.{ ... };` literal whose `invoke` slot points
at the nested trampoline via `xx @__invoke`.
Void-returning shapes emit `typed_fn(block_self.sx_env, args...)`;
non-void emits `return typed_fn(...)`. Per-position arg names
follow `arg0`, `arg1`, ... in declaration order; the typed-fn
cast reconstructs the closure's call signature so the trampoline
hands control back to `sx_fn` with the right argument layout.
`examples/176-build-block-convert.sx` flips green (216/216).
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