* form_multi_dropdown: default drop_multi to select2, keep legacy opt-in
The drop_multi form method now renders a select2 'N Selected' multi dropdown by default instead of the legacy jquery-multiselect widget, closing a gap left from the earlier select2-multi-count work (html_graph/html_tree already opted in explicitly). A caller keeps the legacy jquery-multiselect widget by including 'multiselect' in its field class; the Scheduler fields used by Automation Networks (lib/api_scheduler.php), Data Source Profiles consolidation functions (global_form.php) and the Settings selective-debug/spikekill pickers (global_settings.php) are marked accordingly so their existing per-page .multiselect() initializers keep working unchanged. Also drops the duplicate class concatenation the old code emitted.
* settings: convert selective debug and spikekill template pickers to select2
Mirror of the 1.2.x conversion. Drop the inline .multiselect() initializers for the Logging-tab Selective File/Plugin Debug pickers and the Spikes-tab 'Graph Templates to Spike Kill' picker, and remove their 'multiselect' opt-in class, so applySkin() renders them as the select2 'N Selected' multi dropdown. Each keeps its wording via per-field select_all_text/select_count_text (the select2-multi-count data-attribute mechanism used for graph_template_id): 'Select File(s)'/'Files Selected', 'Select Plugin(s)'/'Plugins Selected', and 'Select Template(s)'/'Templates Selected'. Empty stays the placeholder text (poller_spikekill.php treats no selection as none).
* settings: align array operators in debug/spikekill defs (php-cs-fixer)
* html_form: legacy opt-in strips select2-multi-count; add drop_multi tests; fix stale docblock
When a drop_multi field carries both 'multiselect' and 'select2-multi-count', the legacy opt-in now wins: the select2-multi-count token is stripped so applySkin() does not also decorate the field and collide with the page's own .multiselect() init. Adds tests/Unit/Ui/Html/FormMultiDropdownSelect2Test.php covering the default injection, legacy opt-in, preserved plugin class, no duplication, and the both-token precedence (either order collapses to the legacy widget). Also drops the stale 'Settings selective debug' example from the helper comment since this PR converts those fields to select2.
TheWitness