Release v2.9.0

Release v2.9.0: explicit placeholders win over convention keys; ChoiceType placeholder semantics

Changelog

Fixed

• Explicit placeholders win over the convention key: a root placeholder passed in PHP options (or set in profile defaults, field_types, by_form) is now applied to attr.placeholder even when auto_placeholder is on. Previously the {form}.{field}.placeholder key was already in attr, so the explicit value was silently ignored.
• ChoiceType placeholder semantics: for ChoiceType and its subtypes (EntityType, EnumType, CountryType, UX Autocomplete, custom types whose parent chain reaches ChoiceType, detected through the form registry / type_map), the root placeholder is kept as the empty-option label instead of being moved to attr.placeholder, and placeholder: false is preserved (no empty option; previously it was dropped and Symfony fell back to an empty option / "None" radio). Choice fields no longer get the convention key in attr.placeholder; the generic profile / by_form defaults.placeholder is not applied to them.
• No convention placeholder on checkbox / radio / hidden fields (and subtypes): auto_placeholder no longer adds a meaningless attr.placeholder key to them.

Full Changelog: v2.8.1...v2.9.0