Add placeholder, empty-value, and onConfirm support to DateField

Extracts the DateField changes from #10992: a placeholder shown when the
value is empty, support for an empty initial value (the picker opens at
maximumDate or today), and an onConfirm callback that fires only when the
user commits a date, distinct from onChangeDate which on iOS fires on every
scroll tick.

Adds Storybook examples demonstrating each behavior, and documents comment
style conventions (docblocks for declarations, block syntax for multiline
comments) in CLAUDE.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

CLAUDE.md

@@ -164,12 +164,45 @@ related code together and gives us a better visual cue that there are probably
 other files contained within this "macro" feature, whereas `Component.tsx` on
 its own looks more like a single component file.
 
-### Documentation and Tests Within Features
+### Comments
 
 Comment code when necessary to explain the “why” behind something; avoid
 comments that simply describe the code. Avoid Unicode characters in comments,
 e.g., use `-` not `—`.
 
+Always use docblock (`/** */`) syntax for comments that document a type, type
+member, method, function, or variable. These are the comments a reader expects
+to find attached to a named declaration, and the docblock form makes that intent
+clear and surfaces nicely in editor tooltips.
+
+```tsx
+type DateFieldProps = {
+  /**
+   * An empty string renders the placeholder and opens the picker at today (or
+   * maximumDate, if earlier).
+   */
+  value: string | Date
+}
+
+/**
+ * Date-only input. Accepts a string in the format YYYY-MM-DD, or a Date object.
+ */
+export function DateField() {}
+```
+
+More generally, any multiline comment should use the `/* */` block syntax rather
+than stacked `//` lines. Reserve `//` for short, single-line comments.
+
+```tsx
+/*
+ * The picker requires a valid date, so when value is empty we fall back to
+ * maximumDate (if set) or today.
+ */
+const fallbackDate = maximumDate ? toSimpleDateString(maximumDate) : today
+```
+
+### Documentation and Tests Within Features
+
 For larger features or components, it's helpful to include a README.md file
 within the directory that explains the purpose of the feature, how it works, and
 any important implementation details. The `/Component/index.tsx` pattern lends