fix: fail solidcraft-cli run when a timeline feature fails (#402)

A parameter change can break a feature without failing the command that
made it; parameters.change reports it only in its `errors`. `run` then
exited 0 and exported the bodies as they were before the failing feature:
a shrunk sheet-metal tray whose unfold failed came out folded, without
the holes and cut-outs made after the unfold.

`run` now checks the evaluated timeline before writing anything. Any
feature with an error fails the run with status 1, lists the failing
features on stderr and writes none of the --out and --save files, as a
failing command already does. --allow-errors writes them anyway. `eval`
is unchanged and still reports the errors in its timeline.

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

docs/book/src/guide/getting-started.md

@@ -104,6 +104,11 @@ A script is a list of commands:
 
 ![The bracket example, rendered by solidcraft-cli snapshot](../generated/bracket.png)
 
+A command that fails stops the script. A feature can also fail later, when a parameter change
+rebuilds it; `run` then exits with status 1, lists the failing features and writes none of
+its `--out` and `--save` files, because the bodies lack what those features should have made.
+`--allow-errors` writes them anyway.
+
 Units are millimetres and degrees unless an expression says otherwise (`"1 in"`, `"30 deg"`),
 and Z is up.