ExprTyper.inferType had no `.force_unwrap` arm, so `mk()!` typed as
`.unresolved`. The bind-first form (`v := mk()!; v.field`) worked because
lowerForceUnwrap produces a correctly typed value stored in a slot, but the
chained `mk()!.field` re-derives the receiver type via inferExprType and got
`.unresolved` — the struct-field lookup failed, the field read emitted as
`undef` (garbage), and `mk()!.method()` failed to resolve the method.
Add a `.force_unwrap` arm resolving the operand's optional child type. One
arm fixes every chained form — field, nested `opt!.a.b`, `opt!.method()`
(pointer + value receiver), and `opt![i]` all route receiver typing through
inferExprType.
Regression: examples/0905-optionals-unwrap-field-chain.sx — garbage / compile
error pre-fix, all correct after.
