common/hmaps: Merge slice-valued module config into site config

When a module provides a config key whose value is a slice (e.g.
cascade or permalinks), and the site config declares the same key as a
map with only a merge strategy marker (_merge = 'deep'), the types do
not match and Params.merge silently dropped the module's value, leaving
the site with no effective cascade or permalink config from the module.

Fix Params.merge so that when the destination value is an empty Params
(IsZero — only the _merge key is present) and the source value is a
non-Params type, the source value is used provided the user-declared
merge strategy is not 'none'. This honours the explicit _merge
directive regardless of the surrounding shallow-merge context.

Closes #13869

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

hugolib/cascade_test.go

@@ -544,6 +544,33 @@ cascade:
 	b.AssertFileExists("public/s2/p3/index.html", false)
 }
 
+// Issue 13869
+func TestCascadeSliceFromModule13869(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+theme = 'foo'
+[cascade]
+_merge = 'deep'
+-- content/_index.md --
+---
+title: home
+---
+-- layouts/home.html --
+color: {{ .Params.color }}
+-- themes/foo/hugo.toml --
+[[cascade]]
+[cascade.params]
+color = 'red'
+`
+
+	b := Test(t, files)
+
+	b.AssertFileContent("public/index.html", "color: red")
+}
+
 // Issue 14848
 func TestCascadeParamsLangIssue14848(t *testing.T) {
 	t.Parallel()

hugolib/page_permalink_test.go

@@ -75,7 +75,7 @@ canonifyURLs = %t
 ---
 title: Page
 slug: %q
-url: %q	
+url: %q
 output: ["HTML"]
 ---
 `, test.base, test.uglyURLs, test.canonifyURLs, test.file, test.slug, test.url)
@@ -173,3 +173,31 @@ Some content.
 	b.AssertFileContent("public/myblog/p2/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p2/|Permalink: https://example.com/myblog/p2/|")
 	b.AssertFileContent("public/myblog/p3/index.html", "Single: A page|Hello|en|RelPermalink: /myblog/p3/|Permalink: https://example.com/myblog/p3/|")
 }
+
+// Issue 13869
+func TestPermalinksSliceFromModule13869(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['home','rss','section','sitemap','taxonomy','term']
+theme = 'foo'
+[permalinks]
+_merge = 'deep'
+-- content/books/my-book.md --
+---
+title: My Book
+---
+-- layouts/page.html --
+RelPermalink: {{ .RelPermalink }}
+-- themes/foo/hugo.toml --
+[[permalinks]]
+pattern = '/shelf/:slug/'
+[permalinks.target]
+path = '/books/**'
+`
+
+	b := Test(t, files)
+
+	b.AssertFileContent("public/shelf/my-book/index.html", "RelPermalink: /shelf/my-book/")
+}