refactor: List is slice-backed { items: []T; cap } — directly iterable

items is now a []T slice whose .len IS the live element count (cap = allocated
capacity), so a List iterates directly: `for xs.items (e) { ... }`. A
`len :: (self) -> i64 #get => items.len` accessor keeps `xs.len` reads working;
`.len` WRITES become `.items.len`. List stays 24 bytes (`[]T`=16 + cap=8).

- list.sx: append/ensure_capacity/deinit rewritten for the slice backing. deinit
  guards the free on `cap > 0` (true ownership) and resets via explicit
  ptr=null/len=0 (a `.{}` slice assignment yields a garbage len; `.[]` is the
  empty-slice literal but can't be assigned to a generic []T — both worked around).
- Compiler coupling updated: comptime_vm makeStringList/readStringList write/read
  items as a {ptr,len} fat pointer at field 0 + cap at field 1; control_flow
  listView views an `items: []T` slice (keeps the legacy {[*]T,len} shape too).
- Migrated List `.len` writes to `.items.len` in sched.sx + ui/{render,pipeline,
  glyph_cache} + platform/{sdl3,android,uikit}.
- Snapshots: List's type-table layout changed → ~40 .ir + memory/0800 (items now
  prints as a slice) regenerated; diagnostics/1183 retargeted to a genuine
  many-pointer (xs.items is a slice now). Example memory/0840 locks for-each.

library/modules/ui/glyph_cache.sx

@@ -575,7 +575,7 @@ GlyphCache :: struct {
             return;  // shaped_buf already has the result
         }
 
-        self.shaped_buf.len = 0;
+        self.shaped_buf.items.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.len = 0;
+        self.render_tree.nodes.items.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.len = 0;
+        self.nodes.items.len = 0;
         self.generation += 1;
     }