ludic.ui: <option disabled="{...}"> - a select steps past it, clamps off it and greys the arrow toward it

- The arrows, a press on the select and the pad step past a disabled option (ct_opt_step), wrapping
  as before.
- A value naming a disabled option is shown as the nearest enabled one (the lower of two as near) and
  set to it once the build is done: an event fired while the tree is built is cleared with the frame's,
  so the clamp is queued (ct_clamp_n / _i) and fired after nt_fired_clear.
- The < or > whose next option is disabled is drawn disabled (its part's :disabled), so a cycler shows
  where the options stop being on; it still steps past them.
- ct_build_select moves to controls_opts.ludic with the rest (controls_build.ludic 119 -> 104 lines).

Golden ui_select_disabled: options 3 and 4 disabled, a value of 4 clamps to 2, > is drawn disabled
there, > wraps to 0 and < from 0 steps back to 2. The 38 ui examples pass.

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

packages/ludic.ui/backend.ludic

@@ -53,6 +53,8 @@ export state UiState {
   nt_ev_at: int = 0
   nt_nums: []Val = nt_nums_new()        # fired.ludic: the numbers a scroll or a slider reports, a ring
   nt_num_at: int = 0
+  ct_clamp_n: []UiNode = new []UiNode   # controls_opts.ludic: selects whose value a build clamped
+  ct_clamp_i: []int = new []int
   hd_ev: Val = hd_ev_new()               # hold.ludic: the held control's event, filled in place
   cv_texts: []string = new []string      # units_pieces.ludic: a value's var() pieces, kept
   tx_pieces: [][]string = new [][]string # eval_text.ludic: a text's pieces, a list a depth