refactor: in axis normalize values to arrays, include all span levels

Axis setter now wraps single values in arrays (["A"] instead of "A"),
matching data-viewer. This eliminates Array.isArray branching in
builders, Data slicing/trimming, and format-table.

Also includes the final level in getSpans, fixing a latent crash when
collapseColumns is false on multi-level headers.

Author: lcvriend

src/builders/base-builder.js

@@ -160,10 +160,6 @@ export class BaseTableBuilder {
     getMarginEdgeCols() {
         const cols = []
         this.data.columns.values.forEach((value, icol) => {
-            if (!Array.isArray(value)) {
-                if (this.options.marginLabels.includes(value)) cols.push(icol)
-                return
-            }
             for (let level = 0; level < value.length; level++) {
                 if (!this.options.marginLabels.includes(value[level])) continue
                 if (this.data.columns.spans[level].some(s => s.iloc === icol)) {

src/builders/default-builder.js

@@ -278,7 +278,7 @@ export class DefaultTableBuilder extends BaseTableBuilder {
      * @returns {string} HTML string for the th element
      */
     buildColumnLabel(value, mappedValue, iloc) {
-        const selectedValue = Array.isArray(mappedValue) ? mappedValue.at(-1) : mappedValue
+        const selectedValue = mappedValue.at(-1)
         const attrs = this.data.columns.attrs[iloc]
 
         const attributes = {
@@ -491,7 +491,7 @@ export class DefaultTableBuilder extends BaseTableBuilder {
      * @returns {string} HTML string for the index cell
      */
     buildIndex(value) {
-        const selectedValue = Array.isArray(value) ? value.at(-1) : value
+        const selectedValue = value.at(-1)
         const level = this.data.index.nlevels - 1
         const attrs = this.data.index.attrs[level]
         const formattedValue = this.formatValue(selectedValue, attrs.dtype, attrs.formatOptions ?? this.options)

src/data.js

@@ -111,11 +111,7 @@ export class Data extends EventTarget {
 
         const indexValues = this.index.values
             .slice(startRow, endRow)
-            .map(value =>
-                Array.isArray(value)
-                    ? value.slice(dropLevels)
-                    : value
-            )
+            .map(value => value.slice(dropLevels))
 
         return new Data({
             values: slicedValues,
@@ -156,19 +152,15 @@ export class Data extends EventTarget {
 
             const headIdx = indexValues.slice(0, trimSize)
             const tailIdx = indexValues.slice(-trimSize)
-            const sepIdx = Array.isArray(indexValues[0])
-                ? Array(this.index.nlevels).fill(separator)
-                : separator
+            const sepIdx = Array(this.index.nlevels).fill(separator)
             indexValues = [...headIdx, sepIdx, ...tailIdx]
         }
 
         // Handle columns if needed
         if (this.columns.length > maxColumns) {
             const headCols = columnValues.slice(0, trimSize)
             const tailCols = columnValues.slice(-trimSize)
-            const sepCol = Array.isArray(columnValues[0])
-                ? Array(this.columns.nlevels).fill(separator)
-                : separator
+            const sepCol = Array(this.columns.nlevels).fill(separator)
             columnValues = [...headCols, sepCol, ...tailCols]
 
             values = values.map(row => {

src/settings/format/format-table.js

@@ -322,7 +322,7 @@ export class FormatTable extends HTMLElement {
 
         return `
             <tr data-column-index="${idx}">
-                <td>${Array.isArray(col) ? col.at(-1) : col}</td>
+                <td>${col.at(-1)}</td>
                 <td>${attrs.dtype ?? "-"}</td>
                 <td>${this.getFormatSummary(attrs.formatOptions, attrs.dtype)}</td>
                 <td>