ffi M5.A.next.1d.A: pack impl matching — lock in concrete-only miss

Step 1d lock-in test pinning today's matching behaviour.
`registerParamImpl` records every impl in `param_impl_map` keyed
by `"Proto\x00<arg_mangled>\x00<src_mangled>"`. For a pack impl
`Into(Block) for Closure(..$args) -> $R` the key contains the
pack-shaped closure's mangle (interns with `pack_start = Some(0)`
after 1c.B). At the `xx cl : *Block` site the lookup mangles the
concrete `Closure(s32, bool) -> bool` source and finds nothing —
the existing focused diagnostic fires:

  no `Into(Block) for cl_s32_bool__bool` impl — add a per-signature
  `__block_invoke_<sig>` trampoline + Into impl alongside the
  existing ones in modules/std/objc_block.sx, or declare it in
  your own code

The pack impl is reachable in the file but never considered.

Next commit (1d.B):
- New `param_impl_pack_map` keyed by `"Proto\x00<arg_mangled>"`
  (no src) — populated by `registerParamImpl` when the source
  is pack-shaped.
- `tryUserConversion` walks the pack map on concrete-key miss.
  Pack shape matches when the impl's fixed prefix equals the
  source's matching prefix; the remainder binds to `$args` and
  the source's return type binds to `$R`. Concrete impls win
  over pack impls (specificity).
- `resolveTypeWithBindings` learns the closure_type_expr path
  so the impl body's `self: Closure(..$args) -> $R` substitutes
  to the concrete source closure during monomorphisation.

The `Closure(s32, bool) -> bool` shape is not covered by stdlib
or 96-block-multi-arg's hand-rolled impls, so the pack impl is
the only candidate post-1d.B.

193/193 example tests + `zig build test` green.

tests/expected/155-pack-impl-match.exit

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+1

tests/expected/155-pack-impl-match.txt

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+/Users/agra/projects/sx/examples/155-pack-impl-match.sx:43:21: error: no `Into(Block) for cl_s32_bool__bool` impl — add a per-signature `__block_invoke_<sig>` trampoline + Into impl alongside the existing ones in modules/std/objc_block.sx, or declare it in your own code