ffi checkpoint: step 2a typed pack indexing done

Logs the two step-2a commits (223ec3d lock-in, cd36784 fix):
pack-fn bodies' `args[<int_literal>]` now substitutes the i-th
call-site argument's lowered value directly, propagating the
concrete type through field access and typed coercion.

Out-of-scope follow-ups noted: non-literal comptime indices,
type-position `$args[$i]` (step 3), per-mono mangling, and the
pre-existing nested-comptime-call scope bug (a pack-fn body
calling `print(...)` AND containing `return X;` trips
"unresolved 'result'" — same shape as the issue-0045 family,
worth filing separately if a later slice needs it).

current/CHECKPOINT-FFI.md

@@ -6,6 +6,57 @@ add a test and make it pass — that's two commits).
 
 ## Last completed step
 
+**M5.A.next.2a.B — pack typed indexing: `args[$i]` substitutes call arg**
+(commit `cd36784`, lock-in `223ec3d`).
+
+Step 2 first slice of the pack feature. Pack-fn bodies that
+index the pack via `args[<int_literal>]` now resolve to the
+i-th call-site argument's lowered value directly, propagating
+the call arg's concrete type instead of the boxed `Any` that
+the `[]Any` slice path returns. Lets bodies do field access /
+typed assignments / further indexing on pack elements without
+manual unboxing.
+
+New plumbing in `src/ir/lower.zig`:
+
+- `pack_arg_nodes: ?StringHashMap([]const *const Node)` on
+  Lowering — maps pack param name to the slice of call-site
+  arg AST nodes.
+- `lowerComptimeCall` populates the map when the variadic
+  param is heterogeneous (`is_variadic AND is_comptime` —
+  i.e. `..$args`). Plain `args: ..Any` skips registration so
+  stdlib's `format`/`print` continue boxing through Any.
+  Saved/restored across nested calls.
+- `packArgNodeAt(ie)` returns the call-arg node when an
+  index_expr matches `<pack_name>[<comptime_int_literal>]`
+  with the index in range.
+- `lowerIndexExpr` and `inferExprType`'s index_expr arm both
+  check `packArgNodeAt` first so the concrete type flows
+  through field access and typed-assignment paths.
+
+`examples/156-pack-typed-index.sx` flips from "field 'x' not
+found on type 'Any'" to `7` — `args[0].x` on a struct-typed
+call arg resolves through Point.x correctly.
+
+Out of scope:
+- Non-literal comptime indices (`args[$i]` where `$i` is a
+  comptime expression with binding).
+- `$args[$i]` in type positions (step 3).
+- Per-mono mangling (monomorphisation stays inline-only).
+- Nested comptime calls bug surfaces here too: a pack-fn body
+  that calls `print(...)` AND has a `return X;` trips
+  "unresolved 'result'" because nested comptime inlining
+  loses the scope where stdlib's `#insert build_format`
+  declared `result`. Same class as the
+  `helper :: ($x: s32) -> s64 { print(...); return 42; }`
+  pattern; pre-dates step 2. Worth filing if step 2's later
+  slices need it; today's typed-indexing test exercises only
+  field access and arithmetic, no nested print.
+
+195/195 example tests + `zig build test` green.
+
+---
+
 **issue-0045 fix — inline-return slot for comptime-call bodies**
 (commit `9e78790`, lock-in `3d32ab0`).
 
@@ -433,9 +484,10 @@ plus 2 codegen fixes surfaced along the way.**
 
 ## Current state
 
-- 194/194 example tests pass; `zig build test` green.
+- 195/195 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).
 - issue-0045 (comptime-fn-with-return verifier crash) fixed (9e78790).
 - Chess on macOS / iOS-sim / Android all build and run.