ffi checkpoint: step-2 follow-ups all landed (generic $R / bare args / runtime idx / mixed)

Logs all four step-2 follow-up commits (c917f92, 2e0b97a,
d30d566, 159f898 + their lock-ins). Pack-fns are functionally
complete on the mono path.

Test count: 203/203. Step 6 (stdlib print/format refactor) is
now unblocked from the type-system side; step 5 (generic
Into(Block) impl) still needs step 3 (type-position
$args[$i]) before its trampoline body can be parametric.

iOS-sim chess regression-verified post-step-2b.

Outstanding (not blocking step 3): issue-0046, non-literal
comptime arg mangling.

current/CHECKPOINT-FFI.md

@@ -6,6 +6,44 @@ add a test and make it pass — that's two commits).
 
 ## Last completed step
 
+**M5.A.next.2b.fu1–fu4 — all four step-2b follow-ups landed**
+(commits `c917f92`, `2e0b97a`, `d30d566`, `159f898` + lock-ins).
+
+The four deferred follow-ups from step 2b's initial landing are
+now closed. Pack-fns are functionally complete on the mono
+path; the only remaining gaps are issue-0046 (nested comptime
+calls) and step 3+ work (type-position `$args[$i]` for the
+stdlib payoff).
+
+| FU | Commit | What it does |
+|---|---|---|
+| #2 generic `$R` | `c917f92` + `2e0b97a` | `monomorphizePackFn` infers ret_ty from the body's tail expression / first explicit return when the declared type is generic. New `pack_arg_types` map gives `inferExprType` direct access to per-position pack arg types (avoids the synthesized-ident detour). New `diagPackIndexOOB` emits "pack index N out of bounds: 'pack' has M elements" instead of the misleading "unresolved 'args'" fall-through. |
+| #3 bare `args` | `d30d566` | `materialisePackSlice` builds an `[]Any` slice value for the pack name inside the mono — each pack param boxed via `boxAny`, stored in a stack [N x Any] array. Bare `args` resolves as a runtime slice for forwarding to `[]Any`-typed helpers. Per-position type info is lost via Any boxing; literal-indexed access still routes through `packArgNodeAt` and keeps concrete types. |
+| #4 runtime indexing | `d30d566` (shared) | `args[<runtime_int>]` lowers through the standard slice path against the materialised `[]Any`. Element type `Any` — inherent to runtime indexing into a heterogeneous pack. |
+| #1 mixed comptime+pack | `159f898` | `isPackFn` relaxed to "trailing pack + any non-pack comptime params". `lowerPackFnCall` folds comptime VALUES into the mangle (`__ct_<value>` segment per non-pack comptime); `monomorphizePackFn` binds each comptime non-pack param both as a `comptime_param_nodes` entry AND as a runtime local. `appendComptimeValueMangle` hashes strings, formats int/bool/float for stable mangles. |
+
+New tests:
+- `examples/159-pack-generic-ret.sx` — basic generic `$R`.
+- `examples/160-pack-hetero-ret.sx` — `args[2]` with mixed-type
+  pack returns the third arg's concrete type ("hello").
+- `examples/161-pack-index-oob.sx` — OOB pack index emits a
+  focused diagnostic.
+- `examples/162-pack-bare-args.sx` — bare `args` forwarded to
+  `log_count(items: []Any)`.
+- `examples/163-pack-runtime-index.sx` — `while i < args.len
+  { args[i] }` over a 4-arg pack.
+- `examples/164-pack-mixed-comptime.sx` — `tagged($tag: s32,
+  ..$args)` called with different `tag` values gets distinct
+  monos (`tagged__ct_7__pack_*`, `tagged__ct_9__pack`).
+
+203/203 example tests + `zig build test` green. Step 6's
+stdlib `print` / `format` refactor is now unblocked from the
+type-system side (mixed comptime+pack works); step 5 still
+needs step 3's `$args[$i]` in type positions for the generic
+`Into(Block)` impl body.
+
+---
+
 **M5.A.next.2b — per-call-shape monomorphisation for pack-fns**
 (commit `7989618`).
 
@@ -583,22 +621,29 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Current state
 
-- 197/197 example tests pass; `zig build test` green.
+- 203/203 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 2 done — typed `args[$i]` at literal indices
   (cd36784) + CFG terminator fix (e6d6903) + per-call-shape
   mono (7989618).
+- Pack feature step-2 follow-ups all landed: generic `$R`
+  (c917f92, 2e0b97a), bare `args` + runtime indexing (d30d566),
+  mixed comptime+pack (159f898). Pack-fns are functionally
+  complete on the mono path.
 - 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
   public pack-fn bodies.
+- iOS-sim chess running end-to-end (verified post-step-2b screencap).
 - Chess on macOS / iOS-sim / Android all build and run.
 
 ## Pack feature — next slice options
 
-Step 2 done. Pack-fns monomorphise per call shape; typed
-indexing + arity work end-to-end.
+Step 2 + all four follow-ups done. Pack-fns are functionally
+complete on the mono path: typed indexing, generic returns,
+heterogeneous picks, OOB diagnostics, bare/runtime `args`
+access, mixed comptime+pack — all work.
 
 Step 3 ("Type-reflection intrinsics") is the natural next:
 - `$args[$i]` in TYPE positions (param types, return types,
@@ -612,16 +657,15 @@ Step 4 (`#insert` pack passthrough + `compile_error`) is a
 parser/interp tweak letting builders raise build-time
 diagnostics from inside `#insert` bodies.
 
-Steps 5 and 6 are stdlib refactors gated on step 3's
-type-position substitution landing.
+Step 5 (generic `Into(Block)` impl) and step 6 (stdlib
+`print`/`format` refactor) are the visible payoff slices,
+gated on step 3 (and partially step 4).
 
-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).
+Outstanding items not blocking step 3:
+- issue-0046 (nested-comptime-call + return).
+- Non-literal comptime args in mixed-mode pack-fns (degrades
+  to a `?` mangle segment today — would need comptime
+  evaluation for proper handling).
 
 **M4.0 — context.allocator threading** (4 commits this session):
 - `__sx_allocator: Allocator` prepended at field index 0 of every