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.

examples/diagnostics/1183-diagnostics-many-pointer-to-slice-rejected.sx

@@ -15,10 +15,9 @@ sum :: (s: []i64) -> i64 {
 }
 
 main :: () -> i32 {
-    xs : List(i64) = .{};
-    xs.append(10);
-    xs.append(20);
-    r := sum(xs.items);   // [*]i64 → []i64 — needs xs.items[0..xs.len]
+    a : [4]i64 = .[10, 20, 30, 40];
+    mp : [*]i64 = xx @a[0];   // a genuine many-pointer (carries no length)
+    r := sum(mp);             // [*]i64 → []i64 — rejected; needs mp[0..len]
     print("{}\n", r);
     return 0;
 }

examples/diagnostics/expected/1183-diagnostics-many-pointer-to-slice-rejected.stderr

@@ -1,5 +1,5 @@
 error: a many-pointer '[*]T' does not coerce to a slice '[]T' implicitly (it carries no length) — slice it with a length: ptr[0..len]
-  --> examples/diagnostics/1183-diagnostics-many-pointer-to-slice-rejected.sx:21:10
+  --> examples/diagnostics/1183-diagnostics-many-pointer-to-slice-rejected.sx:20:10
    |
-21 |     r := sum(xs.items);   // [*]i64 → []i64 — needs xs.items[0..xs.len]
-   |          ^^^^^^^^^^^^^
+20 |     r := sum(mp);             // [*]i64 → []i64 — rejected; needs mp[0..len]
+   |          ^^^^^^^