feat: #set property accessors (write counterpart of #get)

A method `name :: (self: *T, value: V) #set { ... }` (or `=> expr;`) is the
write counterpart of a `#get` accessor: `obj.name = rhs` dispatches to it as
`obj.name(rhs)` when no real field matches. Plumbed parallel to `#get`:

- lexer/token `#set`; `FnDecl.is_set` + `Function.is_set`; parsed in the same
  marker slot as `#get` (no return type, exactly self + one value param).
- get+set coexistence: a setter registers/mangles/dispatches under an effective
  `name$set` name (`$` is illegal in sx identifiers, so unmistakable), keeping a
  same-name `#get` under the plain `name`. Resolution is declaration-order-
  independent: a plain read query picks the non-setter, a `name$set` write query
  picks the setter (accessorEffName / accessorNameMatches / structMethodFn).
- write dispatch in lowerAssignment via tryLowerPropertyAssignment: plain assign
  synthesizes `obj.name$set(rhs)`; compound `OP=` is get-modify-set and
  evaluates the receiver EXACTLY ONCE (bound to a synthetic local); read-only
  (#get-only) and write-only (#set-only + compound) emit clear diagnostics; a
  real field of the same name still wins. Multi-assign property targets dispatch
  the setter too (tryLowerPropertyStore, via a pre-lowered-Ref binding).

Payoff: List gains a `len` #set, so `xs.len = n` works; the `.items.len = N`
write workarounds in sched.sx + ui/* + platform/* revert to `xs.len = N`.

issues/0160 records an optional-chain interaction surfaced by the review (a
pre-existing `?T` value-optional read miscompile that blocks getter-through-`?.`).

src/ir/lower.zig

@@ -1746,6 +1746,9 @@ pub const Lowering = struct {
     pub const fnDeclOfRaw = lower_decl.fnDeclOfRaw;
     pub const structDeclOfRaw = lower_decl.structDeclOfRaw;
     pub const structMethodFn = lower_decl.structMethodFn;
+    pub const accessorEffName = lower_decl.accessorEffName;
+    pub const accessorNameMatches = lower_decl.accessorNameMatches;
+    pub const setter_eff_suffix = lower_decl.setter_eff_suffix;
     pub const typeFnAuthor = lower_decl.typeFnAuthor;
     pub const selectedFuncId = lower_decl.selectedFuncId;
     pub const bareAuthorFuncId = lower_decl.bareAuthorFuncId;
@@ -1968,6 +1971,7 @@ pub const Lowering = struct {
     pub const lowerInitBlock = lower_expr.lowerInitBlock;
     pub const getStructFields = lower_expr.getStructFields;
     pub const getAccessorFor = lower_expr.getAccessorFor;
+    pub const getSetterFor = lower_expr.getSetterFor;
     pub const fixupMethodReceiver = lower_expr.fixupMethodReceiver;
     pub const getStructTypeName = lower_expr.getStructTypeName;
     pub const builtinTypeName = lower_expr.builtinTypeName;