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-`?.`).

examples/memory/0840-memory-list-foreach.sx

@@ -2,7 +2,7 @@
 // live element count, so a List is directly iterable with a `for`-each, and
 // `xs.len` reads the live count via a `#get` accessor. Exercises append (incl.
 // a realloc past the initial cap of 4), for-each, parallel for-with-index,
-// empty iteration, direct `for xs` over the List, and truncation via items.len.
+// empty iteration, direct `for xs` over the List, and truncation via `xs.len = 0`.
 #import "modules/std.sx";
 
 main :: () -> i64 {
@@ -33,8 +33,8 @@ main :: () -> i64 {
     while j < xs.len { acc = acc + xs.items[j]; j = j + 1; }
     print("indexed sum={}\n", acc);                       // 210
 
-    // truncate to empty via items.len, then iterate (zero iterations)
-    xs.items.len = 0;
+    // truncate to empty via the `len` #set accessor, then iterate (zero iters)
+    xs.len = 0;
     cnt := 0;
     for xs.items (e) { cnt = cnt + 1; }
     print("after trunc: len={} iters={}\n", xs.len, cnt); // len=0 iters=0