feat(ui): flex-shrink, text that wraps beside its siblings, calc(), order, and 0 as a size

- flex-shrink: a line whose children want more room than it has takes it back from those that
  shrink, in proportion to shrink times size and never below a min size, a fixed size or the
  content. A scroll box shrinks (and scrolls), and a box in a column shrinks as far as the scroll
  boxes inside it let it, so a list in a column takes the room its siblings leave with no height.
  `flex: grow shrink`, and `flex-shrink` by name.
- A row wider than its room measures its texts (and boxes without a width) again in the room the
  rest leave them, so a line wraps beside an icon.
- calc() with + - * / and brackets over px, %, em, rem, vw, vh, plain numbers and var(); a width
  or height keeps its percentage, taken of the room it is given. Lengths lists (padding, margin)
  keep a calc()'s spaces.
- `order` places a box's children without touching the tree.
- width: 0 and height: 0 are 0: an unset size is UI_AUTO now, not 0.
- A component root that is itself a component takes each user's class, style and id in turn (the
  outermost's id wins), and the rules of all their sheets are weighed by specificity together; a
  user's rule wins a tie. The parent's sheet used to override the child's whatever its specificity,
  and a middle component's sheet was lost.
- The default sheet lays .ui-track out as a row, so a range's fill is as tall as its track and a
  checked box's knob goes right; a range's thumb is centred on the fill's end.
- ui_boxes.ludic.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.ui/attr_box.ludic

@@ -24,9 +24,9 @@ var el_s3: float = 0.0
 function el_sides(v: Val) -> void {
   let parts = new []float
   if vl_num(v) { push(parts, ui_px(Value.as_float(v))) } else {
-    let words = lss_split(vl_text(v), 32)
+    let words = el_words(vl_text(v))
     for i in 0 .. len(words) {
-      if len(words[i]) > 0 { push(parts, el_unit(words[i])) }
+      if len(words[i]) > 0 { push(parts, el_word_len(words[i])) }
     }
   }
   while len(parts) < 4 {
@@ -37,11 +37,12 @@ function el_sides(v: Val) -> void {
   el_s2 = parts[2]
   el_s3 = parts[3]
 }
-# a length: a number, 10px, 1.5em, 2rem, 50vw, fill, fit or auto (a percentage is el_pct's)
+# a length: a number, 10px, 1.5em, 2rem, 50vw, calc(...), fill, fit or auto (a percentage is el_pct's)
 function el_len(v: Val) -> float {
   if vl_num(v) { return ui_px(Value.as_float(v)) }
   let s = vl_text(v)
   if s == "fill" { return UI_FILL }
+  if ca_is(s) { return el_calc_len(s) }
   if s == "fit" or s == "auto" or (len(s) > 0 and s[len(s) - 1] == 37) { return 0.0 }
   return el_unit(s)
 }