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.
39a804f25e