fix(types): type force-unwrap so opt!.field chains resolve [0101]

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.

examples/expected/0905-optionals-unwrap-field-chain.exit

@@ -0,0 +1 @@
+0

examples/expected/0905-optionals-unwrap-field-chain.stderr

@@ -0,0 +1 @@
+

examples/expected/0905-optionals-unwrap-field-chain.stdout

@@ -0,0 +1,10 @@
+chain id:  hello
+chain n:   42
+bind id:   hello
+bind n:    42
+meth ptr:  hello
+meth val:  50
+nest tag:  deep
+nest k:    7
+index 0:   10
+index 2:   30