Model: an exhausted id space refuses growth instead of duplicating ids (#407)

alloc_id bumped next_id with saturating_add: a presentation whose
next_id reached u32::MAX (a hostile file can arrive that way;
fix_next_id imports it) handed out that same id to every new slide,
shape, layout and media - duplicate ids, ambiguous references, silent
corruption on every later insert.

alloc_id now returns Option<u32> via checked_add, and every caller
refuses: the engine's insert/duplicate/media/layout commands answer
with a clear error naming the command, paste and duplicate re-id
helpers propagate it, media addition is fallible, and a fresh slide is
Option. The pptx export's synthesized blank layout is guarded the same
way. Loading keeps such a deck viewable: repair re-ids duplicate
slides while it can, and a duplicate it cannot re-id stays as it is -
growth is refused, and only that duplicate keeps its ambiguous id.

Renumbering the loaded graph was rejected: it needs the
cross-document reference remapping that issue #406 tracks.

Tests: model exhaustion (alloc_id None, fresh media refused, a new
slide refused), engine (slide.new, shape.insert, slide.duplicate and
edit.duplicate all refuse; the document holds no duplicate ids),
format repair (an unre-id-able duplicate stays visible).

crates/text/src/tests.rs

@@ -7,7 +7,7 @@ use deckcraft_model::{LayoutType, Presentation, Xfrm};
 fn setup(text: &str, w: f64, h: f64) -> (Presentation, Shape) {
     let mut p = Presentation::default();
     let lay = defaults::layout_of_kind(&p, LayoutType::TitleAndContent).unwrap();
-    let mut s = defaults::new_slide(&mut p, lay);
+    let mut s = defaults::new_slide(&mut p, lay).expect("slide");
     let mut body = s.shapes.remove(1);
     body.xfrm = Some(Xfrm::new(0.0, 0.0, w, h));
     body.text = Some(TextBody::from_text(text));