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

examples/memory/0841-memory-list-len-set.sx

@@ -0,0 +1,21 @@
+// `List(T).len` is a `#get`/`#set` property pair: `xs.len` reads the live
+// element count (delegating to `items.len`), and `xs.len = n` sets it (e.g.
+// `xs.len = 0` to clear the list without freeing its buffer — `cap` and the
+// backing allocation are untouched, so appends reuse the same storage).
+#import "modules/std.sx";
+
+main :: () -> i64 {
+    xs : List(i64) = .{};
+    xs.append(10);
+    xs.append(20);
+    xs.append(30);
+    print("len={} cap={}\n", xs.len, xs.cap);   // len=3 cap=4
+
+    xs.len = 0;                                   // clear via the #set property
+    print("after clear: len={} cap={}\n", xs.len, xs.cap);  // len=0 cap=4
+
+    // The buffer survived the clear — re-append reuses it (cap stays 4).
+    xs.append(99);
+    print("reused: len={} cap={} first={}\n", xs.len, xs.cap, xs.items[0]); // 1 4 99
+    return 0;
+}

examples/memory/expected/0841-memory-list-len-set.exit

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

examples/memory/expected/0841-memory-list-len-set.stderr

@@ -0,0 +1 @@
+

examples/memory/expected/0841-memory-list-len-set.stdout

@@ -0,0 +1,3 @@
+len=3 cap=4
+after clear: len=0 cap=4
+reused: len=1 cap=4 first=99