feat(blog): XSLT/XPath function reference section (Phase 2)

- 19 reference pages under /xslt/functions/ covering:
  xsl:value-of, xsl:for-each, xsl:for-each-group, xsl:function,
  xsl:iterate, last(), position(), count(), concat(), contains(),
  substring(), sum(), key(), current(), string-join(), tokenize(),
  matches(), replace(), format-date()
- Each page: description, parameters table, return value, 2 runnable
  examples (input XML + stylesheet + output), notes, cross-links
- New Hugo section with reference-specific layout (version badges,
  syntax block, "Try in Playground" CTA, parameters table styles)
- Menu entry "Reference" → /xslt/functions/
- CSS: .ref-badge, .ref-syntax, .ref-index-list, .ref-try + table styles

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

site/content/xslt/functions/xpath-sum.md

@@ -0,0 +1,115 @@
+---
+title: "sum()"
+description: "Returns the sum of the numeric values in a node-set or sequence, or a specified zero value for empty sequences."
+date: 2026-04-18T00:00:00Z
+version: "1.0"
+versionLabel: "XSLT 1.0"
+category: "numeric function"
+syntax: "sum(node-set)"
+tags: ["xslt", "reference", "xslt1", "xpath"]
+---
+
+## Description
+
+`sum()` converts each node in a node-set to a number (using the XPath `number()` rules) and returns their total as a double. If any node cannot be converted to a number, its value is treated as `NaN`, and the entire result is `NaN`.
+
+In XSLT 2.0+, `sum()` accepts any sequence of atomic values that can be added together. A second argument may be provided as a **zero value** to return when the sequence is empty (in XPath 1.0, `sum()` on an empty node-set returns `0`).
+
+`sum()` is the standard way to compute totals over repeated elements, such as order amounts, quantities, or scores, without writing an explicit loop.
+
+## Parameters
+
+| Parameter | Type | Required | Description |
+|-----------|------|----------|-------------|
+| `node-set` | node-set / sequence | Yes | The nodes or items to sum. Each is converted to a number. |
+| `zero` | atomic value | No | Value to return if the sequence is empty (XPath 2.0+). Defaults to `0`. |
+
+## Return value
+
+`xs:double` (XPath 1.0) or the type matching the input sequence (XPath 2.0+) — the numeric sum of all items.
+
+## Examples
+
+### Sum all child element values
+
+**Input XML:**
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<invoice>
+  <line amount="29.99"/>
+  <line amount="14.50"/>
+  <line amount="5.00"/>
+</invoice>
+```
+
+**Stylesheet:**
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+  <xsl:output method="text"/>
+
+  <xsl:template match="/invoice">
+    <xsl:text>Total: </xsl:text>
+    <xsl:value-of select="sum(line/@amount)"/>
+  </xsl:template>
+</xsl:stylesheet>
+```
+
+**Output:**
+```
+Total: 49.49
+```
+
+### Conditional sum (sum filtered values)
+
+**Input XML:**
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<sales>
+  <sale region="North" amount="100"/>
+  <sale region="South" amount="250"/>
+  <sale region="North" amount="175"/>
+  <sale region="South" amount="80"/>
+</sales>
+```
+
+**Stylesheet:**
+```xml
+<?xml version="1.0" encoding="UTF-8"?>
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
+  <xsl:output method="xml" indent="yes"/>
+
+  <xsl:template match="/sales">
+    <totals>
+      <north><xsl:value-of select="sum(sale[@region='North']/@amount)"/></north>
+      <south><xsl:value-of select="sum(sale[@region='South']/@amount)"/></south>
+    </totals>
+  </xsl:template>
+</xsl:stylesheet>
+```
+
+**Output:**
+```xml
+<totals>
+  <north>275</north>
+  <south>330</south>
+</totals>
+```
+
+### Empty sequence with fallback (XSLT 2.0)
+
+```xml
+<!-- Returns 0 when there are no discount elements -->
+<xsl:value-of select="sum(discount/@value, 0)"/>
+```
+
+## Notes
+
+- Nodes that cannot be converted to a number (e.g., text like `"N/A"`) produce `NaN`, which propagates through the sum. Validate data or use `number()` with a fallback before summing.
+- `sum()` on an empty node-set returns `0` in XPath 1.0. In XPath 2.0+, the second argument controls this behaviour.
+- For an average, there is no built-in `avg()` in XPath 1.0. Compute it as `sum(nodes) div count(nodes)`. XPath 2.0+ provides `avg()`.
+- `sum()` works on attribute nodes as well as element text nodes; the path `sum(items/item/@qty)` is valid.
+
+## See also
+
+- [count()](../xpath-count)