Remove legacy array logic nodes; migrate + dropdown-ify

Delete the action.accumulate / action.clear node kinds (superseded by the
typed action.array.* nodes). Old panels are migrated transparently on load
(xml.ts migrateLegacyArrayNode): accumulate→array.push, clear→array.clear,
and legacy single-`array` export folded into the JSON `columns` form.

Export CSV column pickers now use array-local dropdowns instead of free-text
+ datalist, matching the action.array.* node inspectors. Removes the now-dead
flow-array-names datalist and legacy single-`array` fallbacks.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

web/src/HelpModal.tsx

@@ -657,7 +657,7 @@ function SectionLogic() {
         {[
           { type: 'Triggers',  desc: 'Button press, threshold crossing, value change, timer/interval, panel loop, and On-open / On-close lifecycle.' },
           { type: 'Logic',     desc: 'AND gate, If (then/else branches), and Loop (count or while).' },
-          { type: 'Actions',   desc: 'Write a value to a signal/variable, Delay, Log (debug), and Accumulate / Export-CSV / Clear for in-memory data arrays.' },
+          { type: 'Actions',   desc: 'Write a value to a signal/variable, Delay, Log (debug), Array push/set/remove/pop/clear on an array local variable, and Export-CSV.' },
           { type: 'Dialogs',   desc: 'Info and Error pop-ups, and a Set-point prompt that asks the user for a number and writes it to a target.' },
         ].map(w => (
           <div key={w.type} class="help-widget-row">