ffi M5.A.next.1d.B: pack impl matching — bind $args + $R per call

Pack-shaped impls (`impl P(...) for Closure(..$args) -> $R`) now
match concrete closure sources at xx resolution time. Concrete
impls keep their priority — pack matching only fires on a
concrete-key miss in `param_impl_map`.

New plumbing in src/ir/lower.zig:

- `PackParamImplEntry` carries the pack-shaped source TypeId plus
  the pack-var and ret-var names extracted from the impl AST's
  `target_type_expr`. `registerParamImpl` detects pack-shaped
  sources via `pack_start != null` on the resolved closure type
  and additionally registers in a new `param_impl_pack_map`
  keyed by `"Proto\x00<arg_mangled>"` (no source suffix).

- `tryUserConversion` re-shapes the concrete lookup so the pack
  path runs on miss. `tryPackImplMatch` walks the pack entries,
  verifies the source's fixed prefix matches the impl's prefix,
  binds the pack-var to the source's tail param TypeIds, binds
  the ret-var (when the impl's return is generic) to the source
  return, and monomorphises the convert method. Mangled name
  stays keyed on the concrete source so distinct call shapes
  monomorphise separately.

- `pack_bindings: ?StringHashMap([]const TypeId)` is saved/
  restored around monomorphisation, mirroring `type_bindings`.

- `resolveClosureTypeWithBindings` handles the closure_type_expr
  node during type resolution: when the closure carries a
  `pack_name` AND `pack_bindings` has a binding for it, the
  bound TypeIds are appended after the fixed prefix and the
  result is a concrete (non-pack) closure type — so the impl
  body's `self: Closure(..$args) -> $R` substitutes to the
  concrete source closure during monomorphisation. Without an
  active binding, the pack shape is preserved.

`examples/155-pack-impl-match.sx` flips from the
"no Into(Block) for cl_s32_bool__bool" lock-in diagnostic to
"pack impl match ok": one user-declared
`impl Into(Block) for Closure(..$args) -> $R` covers a
`Closure(s32, bool) -> bool` source that stdlib has no
hand-rolled impl for. Constructed Block isn't invoked
(invoke=null) — the test exercises only the matching +
monomorphisation, not the trampoline (step 5 of the plan).

Existing concrete-impl paths unchanged: 95-objc-block-noop,
96-objc-block-multi-arg, and stdlib's hand-rolled
`Into(Block) for Closure(bool) -> void` continue to pass through
the concrete map first. Same-file duplicate pack impls
diagnose at registration; cross-module visibility and
multi-pack-impl specificity stay TODOs (matching the deferred
Phase 5 work on the concrete path).

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

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

@@ -1 +1 @@
-1
+0

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

@@ -1 +1 @@
-/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
+pack impl match ok