Manage kits: reconcile the schema on save, and a dialog to repin builds

Placing a palette component no longer edits <schema> straight away. The kit is
noted as pending, its manifest is compiled so the box can be painted, and the
status bar says the kit will be declared on save. The <schema> block is then
reconciled when the file is written, in the one place that can report the whole
kit list the file will carry, with each kit's version and checksum.

What a kit's manifest depends on counts as used too, transitively, so placing a
component can bring in more than one entry: hvac pulls in types, ontrolPion
pulls in ontrolPlatform.

New Kits > Manage Kits... dialog lists every kit in three groups -- needed by
the app, declared but unused, and merely installed -- with the pinned build, the
newest installed build, and an Action dropdown of every build a kit ships.
The tick means the schema declares the kit: grey and fixed for kits the app
needs, unticked to remove, ticked to add at the newest build. "Upgrade all kits
to latest version" moves every declared kit; "Update kits" applies the ticks and
the Action column, additions and removals included.

Every schema change is applied as a batch, then verified: a missing manifest or
a component type that stops resolving reverts the whole block and reports why,
and a kit the app needs cannot be removed. Changes are one undo step, which
recompiles the slot registry in both directions since a repin can change a
component's slots.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

sax_dom.py

@@ -3,9 +3,29 @@
 Pure XML and bit work, deliberately free of Tk so the rules stay testable headless.
 """
 
+import copy
 import xml.etree.ElementTree as ET
 
 
+def snapshot_children(element):
+    """Copies an element's children and leading text, so it can be put back wholesale.
+
+    Used for the <schema> block, which is short enough that copying it whole is easier to
+    trust than tracking which attribute moved and which entry was inserted where.
+    """
+    return [copy.deepcopy(child) for child in element], element.text
+
+
+def restore_children(element, snapshot):
+    """Puts back what snapshot_children captured, whitespace and all."""
+    children, text = snapshot
+    for child in list(element):
+        element.remove(child)
+    element.text = text
+    for child in children:
+        element.append(copy.deepcopy(child))
+
+
 def read_meta_property(meta_str):
     """Parses 32-bit meta property integer fields via structured byte extraction rules."""
     if meta_str is None or not meta_str: