ffi checkpoint: 2a control-flow fix + step 2/3 roadmap

Logs the 2a.C/2a.D pair (6b7a66b + e6d6903) — the if-return
regression in issue-0045's original fix, now resolved via the
SSA "return-done block" pattern.

Step 2 is functionally sufficient for the impl-matching payoff
(step 5 of the plan); per-call-shape monomorphisation deferred
until a real workload demands it.

Step 3 ("Type-reflection intrinsics") flagged as the natural
next slice: `$args[$i]` in type positions unlocks the block-
trampoline body in stdlib's generic Into(Block) impl, which is
the visible payoff of the whole pack feature.

current/CHECKPOINT-FFI.md

@@ -6,6 +6,48 @@ add a test and make it pass — that's two commits).
 
 ## Last completed step
 
+**M5.A.next.2a.D — inline-return uses CFG terminator, not block_terminated**
+(commit `e6d6903`, lock-in `6b7a66b`).
+
+Fixes a control-flow regression in issue-0045's original fix
+(commit `9e78790`). The original set `block_terminated = true`
+after each inline `return X;` to skip dead code in the inlined
+body — but the flag leaked past structured control flow. A
+body shaped `if cond { return X; }; return Y;` had its
+trailing `return Y;` short-circuited at `lowerBlockValue`'s
+`if (self.block_terminated) return null;` check. For the false-
+condition path, the slot was never written → load read
+uninitialised stack memory.
+
+Reshaped to the classical SSA "return-done block" pattern:
+
+- `InlineReturnInfo` gains a `done_bb: BlockId` field — a
+  fresh basic block allocated by `lowerComptimeCall` per
+  comptime-call instance.
+- `lowerReturn`'s inline path stores into the slot, drains
+  defers, and emits `br done_bb`. The basic block's terminator
+  carries the "no fall-through" signal; the
+  `block_terminated` flag is no longer touched.
+- `lowerComptimeCall` lowers the body, then unconditionally
+  switches to done_bb and loads the slot. Tail-expression
+  bodies (rare when has_return is true) get a synthetic store
+  + br so the CFG stays well-formed.
+
+`examples/157-pack-if-return.sx` flips from `8354116000` (the
+uninit slot load) to `-1`. A three-way `classify(..$args)`
+smoke confirms multi-return-path bodies work for any of the
+three branches; defer-with-return still fires the defer at the
+correct exit.
+
+issue-0046 (nested `print` inside `..$args` body that also
+`return`s) is unrelated to this fix and is still open — same
+two faces as filed: interp panic for plain comptime, "unresolved
+'result'" for pack-fn.
+
+196/196 example tests + `zig build test` green.
+
+---
+
 **M5.A.next.2a.B — pack typed indexing: `args[$i]` substitutes call arg**
 (commit `cd36784`, lock-in `223ec3d`).
 
@@ -484,13 +526,42 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Current state
 
-- 195/195 example tests pass; `zig build test` green.
+- 196/196 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[$i]` at literal indices (cd36784).
+- Pack feature step 2a done — typed `args[$i]` at literal indices
+  (cd36784) + control-flow CFG terminator fix (e6d6903).
 - 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.
 - Chess on macOS / iOS-sim / Android all build and run.
 
+## Pack feature — next slice options
+
+Step 2 ("Runtime indexing + mono expansion") is functionally
+sufficient for the impl-matching payoff (step 5) thanks to step
+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:
+- `$args[$i]` in TYPE positions (param types, return types,
+  fn-pointer types, struct fields).
+- `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
+diagnostics from inside `#insert` bodies.
+
+Steps 5 and 6 are stdlib refactors gated on step 3's
+type-position substitution landing.
+
 **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