fix: struct-literal → optional coercion + #get through optional chain (issue 0160)

Two fixes for optional interactions surfaced by the #set/#get review. The
original issue 0160 mis-diagnosed (A) as an optional-chain bug; the chain works
fine for real fields. The actual bugs:

(A) A bare struct literal `.{ ... }` against an optional target `?T` was built
into the optional's {payload, has_value} layout instead of the inner T, then
re-wrapped — corrupting the value (a multi-field payload's first field clobbered
by the has_value flag, or a `?T` arg silently null) or failing LLVM
verification. lowerStructLiteral now builds the inner T, materializes it, and
wraps via coerceToType; lowerVarDecl's previously-UNCONDITIONAL optional wrap is
guarded so an already-`?T` value isn't double-wrapped. Fixed across var-decl,
arg, return, nested field, reassignment, and array-element contexts.

(B) `#get` accessors are now reachable through an optional chain (`obj?.getter`):
lowerOptionalChain dispatches the getter via a synthetic receiver, and
expr_typer types `obj?.getter` through a shared getterReturnTypeOnDeref helper
(handles `?T` and `?*T`, value and pointer optionals, and generic-instance
getters like List.len). The `#set` write side through `?.` is intentionally left
matching real-field behavior (optional-chain assignment unsupported).

Regression tests: examples/optionals/0906 (struct-literal → optional) and 0907
(accessor through chain). issues/0160 marked RESOLVED with the corrected root
cause.

src/ir/lower.zig

@@ -1972,6 +1972,7 @@ pub const Lowering = struct {
     pub const getStructFields = lower_expr.getStructFields;
     pub const getAccessorFor = lower_expr.getAccessorFor;
     pub const getSetterFor = lower_expr.getSetterFor;
+    pub const getterReturnTypeOnDeref = lower_expr.getterReturnTypeOnDeref;
     pub const fixupMethodReceiver = lower_expr.fixupMethodReceiver;
     pub const getStructTypeName = lower_expr.getStructTypeName;
     pub const builtinTypeName = lower_expr.builtinTypeName;