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/engine/src/cmd/mod.rs

@@ -152,6 +152,10 @@ pub fn find_command(id: &str) -> Option<&'static CommandSpec> {
 
 // ---------- param helpers ----------
 
+/// The deck has used every object id (a hostile file can arrive that way): it cannot grow, and
+/// growing would duplicate ids (#407).
+pub(crate) const NO_ID_SPACE: &str = "the presentation has used every object id and can hold no more";
+
 pub(crate) fn bad(cmd: &str, msg: impl Into<String>) -> EngineError {
     EngineError::BadParams { cmd: cmd.into(), msg: msg.into() }
 }