ffi checkpoint: step 2b done — per-call-shape pack monomorphisation

Logs commit 7989618 (the 2b shift from inline expansion to real
shared mono fns) and updates the test count to 197/197.

Pack feature step 2 is fully done — typed indexing, control-flow,
and per-call-shape mono all land. Step 3 (type-reflection
intrinsics) is the next slice and unlocks the stdlib payoff
(generic Into(Block) for Closure(..$args) -> $R) by enabling
$args[$i] substitution in type positions.

Documented follow-ups: mixed $fmt+..$args shapes, generic $R
binding, bare args reference, runtime indexing, and issue-0046
fix all remain deferred — none block step 3.

current/CHECKPOINT-FFI.md

@@ -6,6 +6,63 @@ add a test and make it pass — that's two commits).
 
 ## Last completed step
 
+**M5.A.next.2b — per-call-shape monomorphisation for pack-fns**
+(commit `7989618`).
+
+Pack-fns (detected by `isPackFn(fd)` — last param is the only
+comptime param AND is variadic) now emit ONE shared mono per
+unique call-site signature. Repeat calls with the same
+arg-type tuple share the mono; distinct shapes get distinct
+symbols. Pre-2b each call inlined a fresh body copy into the
+caller's basic block.
+
+`examples/158-pack-mono-dedup.sx` confirms end-to-end:
+`count(), count(1), count(2), count(1,2,3), count("x", true)`
+produces `0 1 1 3 2` at runtime AND emits exactly 4 monos in
+IR — the two `s64` calls share one mono.
+
+Plumbing in `src/ir/lower.zig`:
+- `isPackFn(fd)` — true when the only comptime param is a
+  trailing pack. Mixed `($fmt, ..$args)` shapes stay on the
+  inline `lowerComptimeCall` path (different substitution
+  mechanism for the non-pack comptime; deferred).
+- `lowerPackFnCall` — builds mangled name
+  `<fn_name>__pack__<arg_types>`, cache-checks
+  `lowered_functions`, calls `monomorphizePackFn` on miss,
+  emits a direct call.
+- `monomorphizePackFn` — mirrors `monomorphizeFunction`'s
+  save/restore + param/scope setup, with N synthesised pack
+  params (`__pack_<name>_<i>`) and AST-ident substitution
+  installed via `pack_arg_nodes`. `pack_param_count` makes
+  `args.len` resolve to the comptime N via new intercepts in
+  `lowerFieldAccess` + `inferExprType`. `inline_return_target`
+  is nulled out so the mono body emits real `ret X` instead
+  of the inline-slot routing — it's a real fn now.
+- Routed at three call sites: each `hasComptimeParams(fd)
+  → lowerComptimeCall` now first checks `isPackFn` and routes
+  to `lowerPackFnCall` when true.
+
+Lifetime gotcha caught and fixed: `Function.init` stores
+`params.items` by reference (no copy). The local
+`ArrayList(Function.Param)` must NOT be deinit'd — matches the
+leak convention in `monomorphizeFunction`. Symptom of getting
+this wrong: `0xAAAAAAAA` poison-pattern TypeIds in
+`coerceCallArgs`.
+
+`examples/156-pack-typed-index.sx` (typed indexing) and
+`examples/157-pack-if-return.sx` (control flow) continue to
+pass unchanged on the new path.
+
+Out of scope (deferred to follow-up slices):
+- Mixed `$fmt + ..$args` shapes.
+- Generic `$R` return types.
+- Bare `args` reference (passing the pack-slice as a whole).
+- `args[<runtime_int>]` non-literal indexing.
+
+197/197 example tests + `zig build test` green.
+
+---
+
 **M5.A.next.2a.D — inline-return uses CFG terminator, not block_terminated**
 (commit `e6d6903`, lock-in `6b7a66b`).
 
@@ -526,11 +583,12 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Current state
 
-- 196/196 example tests pass; `zig build test` green.
+- 197/197 example tests pass; `zig build test` green.
 - Phase 3.0/3.1/3.2 + M1.0–M1.3 + M2.1–M2.3 + M3 + M4.0 + M4.A all landed.
 - Pack feature step 1 done (1c.A → 1d.B; commits bb6eca6 → 08feb60).
-- Pack feature step 2a done — typed `args[$i]` at literal indices
+- Pack feature step 2 done — typed `args[$i]` at literal indices
-  (cd36784) + control-flow CFG terminator fix (e6d6903).
+  (cd36784) + CFG terminator fix (e6d6903) + per-call-shape
+  mono (7989618).
 - issue-0045 (comptime-fn-with-return verifier crash) fixed (9e78790).
 - issue-0046 (nested-comptime-call + return) FILED, not blocking
   next slices since builders run inside `#insert`, not inside
@@ -539,21 +597,16 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Pack feature — next slice options
 
-Step 2 ("Runtime indexing + mono expansion") is functionally
+Step 2 done. Pack-fns monomorphise per call shape; typed
-sufficient for the impl-matching payoff (step 5) thanks to step
+indexing + arity work end-to-end.
-1d.B's per-mono mangling on `<src>.convert__<dst>` keys.
-Remaining within step 2 is real per-call-shape monomorphisation
-(today pack-fns inline at every call site) — performance/IR-size
-concern, not correctness. Defer until a real workload demands it.
 
-Step 3 ("Type-reflection intrinsics") is the natural next slice:
+Step 3 ("Type-reflection intrinsics") is the natural next:
 - `$args[$i]` in TYPE positions (param types, return types,
-  fn-pointer types, struct fields).
+  fn-pointer types, struct fields). Unlocks the block-trampoline
+  body in stdlib's generic `Into(Block) for Closure(..$args) ->
+  $R` impl — the body needs `typed_fn : (*void, $args[0],
+  $args[1]) -> $R = xx ...` per-position type interpolation.
 - `type_name`, `type_eq`, `has_impl` comptime intrinsics.
-- Unlocks the block-trampoline body in stdlib's generic
-  `Into(Block) for Closure(..$args) -> $R` impl — the body
-  needs `typed_fn : (*void, $args[0], $args[1]) -> $R = xx ...`
-  per-position type interpolation.
 
 Step 4 (`#insert` pack passthrough + `compile_error`) is a
 parser/interp tweak letting builders raise build-time
@@ -562,6 +615,14 @@ diagnostics from inside `#insert` bodies.
 Steps 5 and 6 are stdlib refactors gated on step 3's
 type-position substitution landing.
 
+Pack feature follow-ups (deferred):
+- Mixed `$fmt + ..$args` shapes through the mono path.
+- Generic `$R` return type binding.
+- Bare `args` reference (materialise an `[]Any` slice on demand
+  inside the mono).
+- `args[<runtime_int>]` non-literal indexing.
+- issue-0046 fix (nested-comptime-call + return).
+
 **M4.0 — context.allocator threading** (4 commits this session):
 - `__sx_allocator: Allocator` prepended at field index 0 of every
   sx-defined class's state struct