selfhost/cmd/wcc/check+test: fold size(T)/align(T)/offset(e.f) at check time

Mirror cstage cmd/wcc/check.c:907-960. Three typed-builtin
intercepts that cstage already had:

- size(T) — folds to a literal integer at check time from a
  newly-introduced astsize walker over the type AST. Mirrors the
  size computation in cstage resolve_type at check.c:286-528.
- align(T) — same, via astalign.
- offset(e.f) — folds the byte offset of field f in e's struct
  type via astoffset. Peels exactly one N_TPTR for `p.field`.

seedprimitives registers the three names as SK_FN nil; exprtype's
N_CALL arm gates on a same-module shadow check (per #23 alloc
precedent) and consumes the parser-planted type-expression arg.
The fold is in-place — foldtointlit mutates N_CALL into N_INTLIT
so cgen sees a plain integer. resolvewalk's N_CALL trigger
invokes exprtype so the fold fires from non-let contexts too
(e.g. inside `if (size(T) != …)`).

selfhost/test/smoke.ww gains a probe-8 block: size/align/offset
assertions across str, primitive widths, ptrs, slices, and
two structs (`point`, `mixalign`) covering both no-padding and
i8+i64 natural-align padding cases.

Known divergences NOT in #42 scope:
- size((*T|void)) ≠ 8 on the cstage nullable-ptr fold (#13 family,
  unreachable through current grammar).
- 8B-struct bare-let zero-init wwstage skip vs cstage emit (#59).
- Same-module shadow gate added here, cstage has none — sibling
  shape to #26 (free/append/len gates).

Closes the original chain that started with the user's call to
fix the structural debt — six precondition fixes (#51, #52, #53,
#55, #56, #50) landed before this fold could safely live in the
check pass. Unblocks #43 (sweep literal 16s → size(str)) and #1
(str → 24B becomes one line).

.gitignore

@@ -12,6 +12,7 @@
 /memiotest
 /trypromote
 /tagged_ptr_ret
+/smoke
 
 # Per-module build artifacts. The .combined.ww files under selfhost/
 # are intentionally tracked — they're frozen bootstrap inputs.