docs(layouts): add header comments to undocumented partials

Add `{{/* ... */}}` header comments to 25 partials that had no documentation,
enabling `pnpm gen:docs` to generate their reference docs. Also fix
base/head/index.html leading blank line that prevented parser matching.

Groups covered: base/, feed/, function/, home/, init/, plugin/, single/, store/, (root).

layouts/_partials/single/expiration-reminder.html

@@ -1,3 +1,11 @@
+{{- /*
+  Content expiration reminder partial.
+
+  Displays a note or warning admonition when the page's last modification date
+  exceeds configurable day thresholds (default: 90 days for note, 180 days for warning).
+  Skipped for password-protected pages.
+*/ -}}
+
 {{- $expirationReminder := dict "Page" . "Key" "expiration_reminder" | partial "function/param.html" | default dict -}}
 
 {{- if $expirationReminder.enable | and (not .Params.password) -}}

layouts/_partials/single/post-author.html

@@ -1,3 +1,10 @@
+{{- /*
+  Post author display partial.
+
+  Renders the author name with optional avatar and link.
+  Stores the resolved author map in the page store for use by other partials.
+*/ -}}
+
 {{- $author := partial "function/get-author-map.html" .Params.author -}}
 {{- .Store.Set "author" $author -}}
 {{- $optim := slice 

layouts/_partials/single/post-included-in.html

@@ -1,3 +1,10 @@
+{{- /*
+  Post categories and collections display partial.
+
+  Renders category and collection links with icons for the current page.
+  Displays categories only, collections only, or both depending on which are present.
+*/ -}}
+
 {{- $categories := slice -}}
 {{- range .GetTerms "categories" -}}
   {{- $categories = $categories | append (

layouts/_partials/single/reward.html

@@ -1,3 +1,10 @@
+{{- /*
+  Post reward/donation panel partial.
+
+  Reads the reward config from front matter and site defaults,
+  then delegates rendering to plugin/reward.html.
+*/ -}}
+
 {{- $reward := dict "Page" . "Key" "reward" | partial "function/param.html" -}}
 {{- $author := .Store.Get "author" -}}
 {{- $options := dict "Reward" $reward "Id" "fi-reward" "Author" $author.name -}}