Sibling to the silent-fallback-defaults rule. Catch-all `else`
branches that pass a value through unchanged, write a default width,
or swallow errors into a zero-init are the same class of bug — a
case the implementer didn't think of corrupts data silently.
Both bites this session:
- `storeAtRawPtr` writing 8 bytes regardless of IR type (fixed by
threading val_ty through inst.Store).
- `.deref` else-arm returning val unchanged (now errors loudly for
raw pointers).
- comptime init catch swallowing the error into `.void_val`.
Preferred order: implement the arm in the same step. If plumbing is
out of scope, bail loudly with `bailDetail(comptime msg)` and leave a
one-line comment about what's needed. Width/type/layout info that's
ambiguous from the Value tag belongs in the IR op struct, not in a
"this case is usually 8 bytes" shortcut.
