Add SiloMetadataEditor widget that opens in an MDI subwindow when the
user double-clicks the Metadata node in the Silo tree. Supports editing
lifecycle state, tags (add/remove chips), and schema-defined fields with
type-inferred widgets (QCheckBox, QDoubleSpinBox, QLineEdit).
Changes:
- silo_viewers.py: SiloMetadataEditor with dirty tracking, Save/Reset
buttons, unsaved-changes close guard, and tag chip management
- silo_objects.py: mark_dirty()/is_dirty()/clear_dirty() on proxy
- kc_format.py: fix entries=None before hooks; _metadata_save_hook
writes dirty RawContent back to silo/ cache on document save
Closes#39