Align: Align to Artboard uses the active artboard

Align to Artboard aligned the selection to the artboard under its
center, or to the first artboard when no artboard was under the
center. A press with a selection tool inside an artboard makes that
artboard the active one (#693). With the Selection tool, a
Shift-click on an empty spot inside an artboard also makes it active
and keeps the selection. Align to Artboard did not use the active
artboard.

object.align takes artboard, a 0-based artboard index, and uses it
when its to param is "artboard". The app passes its active artboard,
the one the Artboards panel highlights, to each object.align it runs
that names none, as it does for Paste in Place, Paste in Front, Paste
in Back (#693) and Select › All on Active Artboard (#1006). MCP calls
in remote mode also get it. Without artboard (headless MCP and the
CLI), or with an index the document doesn't have, object.align aligns
to the artboard under the center of the selection, or to the first
artboard when no artboard is under the center.

command.batch takes artboard too, and the app passes it the active
artboard. The batch passes it on to each step of these five commands
that names none, when the step runs. When the document a step runs in
has no artboard at that index (for example, after an earlier step
deleted the active artboard when it was the last one), the step gets
that document's last artboard. A step in a document that an earlier
step opened, created or switched to gets the same index, not that
document's own active artboard.

With Align to Selection, one selected object aligned from the Align
panel, the Control bar or the Properties panel is aligned to the
active artboard too, unless it is the key object.

Recorded actions leave out the artboard that the app passes to these
commands and to the batch. The Actions panel plays an action as a
batch. Its Align to Artboard, All on Active Artboard and in-place
paste steps that name no artboard use the artboard that is active
when it plays, and the default Center on Artboard action now centers
on the active artboard. In-place paste steps recorded since #693 and
before this change still paste onto the artboard that was active when
they were recorded.

Tests cover object.align with and without artboard on three artboards;
a Shift-click that makes an artboard active, after which Align to
Artboard aligns to that artboard but keeps an artboard the call names;
a batch that passes its artboard to each step when the step runs,
after a step deletes the last artboard and in a new document; recorded
actions that leave the artboard out; and recorded Align to Artboard
and Paste in Place steps and the default Center on Artboard action,
which play on the artboard that is active when they play.

Docs: docs/mcp.md (Align to Artboard), docs/target-app-parity.md.

crates/engine/src/cmd/object.rs

@@ -159,7 +159,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Align",
             ["Window", "Align"],
             None,
-            "{horizontal?: \"left\"|\"center\"|\"right\", vertical?: \"top\"|\"center\"|\"bottom\", to?: \"selection\"|\"artboard\"|\"key\" (default: the key object when the selection has one, select.key, else the selection), bounds?: \"preview\"|\"geometric\" (default: the Use Preview Bounds preference; preview bounds take in strokes)}",
+            "{horizontal?: \"left\"|\"center\"|\"right\", vertical?: \"top\"|\"center\"|\"bottom\", to?: \"selection\"|\"artboard\"|\"key\" (default: the key object when the selection has one, select.key, else the selection), artboard?: index (0-based; with to: \"artboard\", the artboard to align to; the app passes the active one; default: the artboard under the center of the selection, else the first), bounds?: \"preview\"|\"geometric\" (default: the Use Preview Bounds preference; preview bounds take in strokes)}",
             has_selection,
             align
         ),
@@ -965,8 +965,12 @@ fn reference_rect(s: &Session, p: &Value, ids: &[NodeId], preview: bool) -> Resu
     match align_to(s, p) {
         Some("artboard") => {
             let b = st.doc.bounds_of(ids, preview).unwrap_or_default();
+            // Align to Artboard uses artboard `artboard` (the app passes the active one). Without
+            // it, or with an index the document doesn't have, it uses the artboard under the center
+            // of the selection, or the first one when no artboard is there.
+            let named = p.get("artboard").and_then(Value::as_u64).and_then(|i| usize::try_from(i).ok()).and_then(|i| st.doc.artboards.get(i));
             let i = st.doc.artboard_at(b.center()).unwrap_or(0);
-            Ok(st.doc.artboards.get(i).map(|a| a.rect).unwrap_or(b))
+            Ok(named.or(st.doc.artboards.get(i)).map(|a| a.rect).unwrap_or(b))
         }
         Some("key") => {
             let k = st.selection.key.ok_or_else(|| EngineError::Other("no key object".into()))?;