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-`?.`).

readme.md

@@ -313,6 +313,15 @@ List :: struct ($T: Type) {
     // `#get` property accessor: read via no-paren field syntax (`xs.len`),
     // not `xs.len()`. Takes only `self`; a real field of the same name wins.
     len :: (self: *List(T)) -> i64 #get => self.items.len;
+
+    // `#set` property accessor: the write counterpart, invoked via field-assign
+    // (`xs.len = n`) rather than `xs.len(n)`. Takes `self` + one value param and
+    // returns void. A property may have BOTH a `#get` and `#set` of the same
+    // name (reads pick the getter, writes the setter); compound assignment
+    // (`xs.len += 1`) reads via `#get` then writes via `#set`. Writing a
+    // `#get`-only property is a "read-only" compile error; a real field of the
+    // same name still wins.
+    len :: (self: *List(T), v: i64) #set { self.items.len = v; }
 }
 ```