Adds the generic `impl Into(Block) for Closure(..$args) -> $R`
in `library/modules/std/objc_block.sx` alongside the existing
hand-rolled `Closure() -> void` and `Closure(bool) -> void`
impls. The convert body is a single
`#insert build_block_convert($args, $R);` — per-call-shape
monomorphisation re-runs the builder so each closure shape gets
its dedicated nested `callconv(.c)` trampoline + Block literal.
The impl-mono path threads pack types through
`pack_bindings[args]` and the single-type return through
`type_bindings[R]`. Both need to be visible to the body's
`$args` / `$R` expression-position references — the existing
lowering only consulted `pack_arg_types` (set by pack-fn mono,
not by tryPackImplMatch). Two small extensions:
- `lowerExpr`'s `.comptime_pack_ref` arm now consults
`pack_arg_types` → `pack_bindings` → `type_bindings` in order,
treating a `type_bindings` hit as a single `const_type(T)`
value rather than the slice form.
- `resolveTypeArg` grows a `.comptime_pack_ref` arm that maps
the same name through `type_bindings` so type-arg positions
(e.g. inside `type_name(...)` in the builder body) resolve
the bound single Type.
- `type_bridge.isTypeShapedAstNode` lists `comptime_pack_ref`
and `pack_index_type_expr` as type-shaped so
`buildTypeBindings`'s strategy-1 explicit-arg path picks
them up when calling a `$T: Type`-generic fn.
`examples/177-generic-into-block.sx` flips green: a
`Closure(s64, s64) -> void` (no hand-rolled impl) is converted
through the generic impl, its block invoked via a typed
`callconv(.c)` fn-pointer, and the closure's side effects land
in the host globals. Hand-rolled impls remain for `()` and
`(bool)` shapes; 5.3 deletes those once a focused test covers
their behaviour through the generic path. Suite at 217/217.
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