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-`?.`).

library/modules/platform/android.sx

@@ -400,7 +400,7 @@ impl Platform for AndroidPlatform {
     }
 
     poll_events :: (self: *AndroidPlatform) -> []Event {
-        self.events.items.len = 0;
+        self.events.len = 0;
         sx_android_drain_touches(self, @self.events);
         result : []Event = ---;
         result.ptr = self.events.items;

library/modules/platform/sdl3.sx

@@ -144,7 +144,7 @@ impl Platform for SdlPlatform {
     }
 
     poll_events :: (self: *SdlPlatform) -> []Event {
-        self.events.items.len = 0;
+        self.events.len = 0;
         sdl_event : SDL_Event = .none;
         while SDL_PollEvent(@sdl_event) {
             if sdl_event == {

library/modules/platform/uikit.sx

@@ -379,7 +379,7 @@ impl Platform for UIKitPlatform {
         result : []Event = ---;
         result.ptr = self.events.items;
         result.len = self.events.len;
-        self.events.items.len = 0;
+        self.events.len = 0;
         result
     }
 

library/modules/std/list.sx

@@ -15,6 +15,10 @@ List :: struct ($T: Type) {
 
     // No-paren read accessor: `xs.len` → the live element count.
     len :: (self: *List(T)) -> i64 #get => self.items.len;
+    // Write accessor: `xs.len = n` sets the live count (e.g. `xs.len = 0` to
+    // clear without freeing). Mirrors the `#get` above; the buffer / `cap` are
+    // untouched, so `n` must be `<= cap`.
+    len :: (self: *List(T), v: i64) #set { self.items.len = v; }
 
     append :: (list: *List(T), item: T, alloc: Allocator = context.allocator) {
         if list.items.len >= list.cap {

library/modules/std/sched.sx

@@ -647,7 +647,7 @@ remove_timer :: (self: *Scheduler, idx: i64) {
         self.timers.items[i] = self.timers.items[i + 1];
         i = i + 1;
     }
-    self.timers.items.len = self.timers.items.len - 1;
+    self.timers.len = self.timers.len - 1;
 }
 
 // Remove a pending sleep timer referencing fiber `f`, if any. A fiber has at
@@ -676,7 +676,7 @@ remove_io_waiter :: (self: *Scheduler, idx: i64) {
         self.io_waiters.items[i] = self.io_waiters.items[i + 1];
         i = i + 1;
     }
-    self.io_waiters.items.len = self.io_waiters.items.len - 1;
+    self.io_waiters.len = self.io_waiters.len - 1;
 }
 
 // Remove a pending fd-waiter referencing fiber `f`, if any. A fiber has at most

library/modules/ui/glyph_cache.sx

@@ -575,7 +575,7 @@ GlyphCache :: struct {
             return;  // shaped_buf already has the result
         }
 
-        self.shaped_buf.items.len = 0;
+        self.shaped_buf.len = 0;
         if text.len == 0 { return; }
 
         if is_ascii(text) {

library/modules/ui/pipeline.sx

@@ -141,7 +141,7 @@ UIPipeline :: struct {
 
         // Reset render_tree nodes (backing is stale after arena reset)
         self.render_tree.nodes.items = null;
-        self.render_tree.nodes.items.len = 0;
+        self.render_tree.nodes.len = 0;
         self.render_tree.nodes.cap = 0;
 
         push Context.{ allocator = xx build_arena, data = context.data } {

library/modules/ui/render.sx

@@ -47,7 +47,7 @@ RenderTree :: struct {
     }
 
     clear :: (self: *RenderTree) {
-        self.nodes.items.len = 0;
+        self.nodes.len = 0;
         self.generation += 1;
     }