on_update reran toggle_hide_tax_id, toggle_editable_rate_for_bundle_items
and toggle_discount_accounting_fields on every save, rewriting 11
property setters and clearing the meta cache of five doctypes.
Gate each toggle on has_value_changed. Fresh installs save the settings
with pure defaults (set_single_defaults), so the gated-off state must
match the JSON schema: align sales_invoice.json and
sales_invoice_item.json with the values every saved site already has —
tax_id printed when hide_tax_id is off, discount accounts hidden while
discount accounting is disabled. Packed Item rate already matches.
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