feat!: add BorderedTextField; migrate fields to ControlPresentation

[pawelgrimm]

Short description

Follows up on #1037. Migrates the existing field components to compose ControlPresentation and introduces a new public sibling, BorderedTextField, that replaces the outlined-label-inside layout previously offered by <TextField variant="bordered">.

New public component

• BorderedTextField — outlined text-field layout: the label sits inside a rounded chrome above the input; optional endSlot rendered as a full-height side column. Composes BaseField + OutlinedControlContainer (the private chrome primitive added in feat: add ControlPresentation primitive #1037). Sibling to TextField, not a variant of it.

Field migrations

• TextField drops variant and endSlotPosition. Inline wrapper JSX is replaced with <ControlPresentation>. Local handleClick and useMergeRefs are removed (CP/OCC handle click-to-focus).
• SelectField drops variant. The absolutely-positioned chevron moves into CP's endSlot. selectWrapper chrome CSS goes away — CP owns it.
• TextArea drops variant. Wraps the <textarea> in OutlinedControlContainer directly (not CP — multi-line layout doesn't fit CP's fixed 32px single-row chrome). Auto-expand grid trick stays scoped to TextArea.
• PasswordField unchanged in source but inherits the TextField refactor automatically (no more variant).

BaseField scoped down

• Removes variant, BaseFieldVariantProps, the supportsStartAndEndSlots discriminated union, endSlot, endSlotPosition, and the bordered outer container rendering.
• After this PR, BaseField is pure form-field scaffolding: id generation, ARIA wiring (aria-describedby / aria-invalid), optional label, character count state machine, message rendering, Stack layout, maxWidth / hidden.

Breaking changes

feat!: — major version bump. Migration is mechanical:

• <TextField variant="bordered" .../> → <BorderedTextField .../>. Drop startSlot, endSlotPosition, and characterCountPosition="inline" if used — none of these are supported in the outlined layout. If you need a leading icon, use TextField (default). If you need a side action, use BorderedTextField's endSlot (full-height).
• <TextField variant="default" .../> → drop the variant prop (no-op rename — default was always the default).
• <SelectField variant="bordered" .../> → drop the variant. No BorderedSelectField is shipped (no demand surfaced).
• <TextArea variant="bordered" .../> → drop the variant. No BorderedTextArea is shipped.
• Custom components using BaseField directly: drop variant, supportsStartAndEndSlots, endSlot, endSlotPosition if used. Render slot rows via ControlPresentation instead.

What's not here

• Anything outside the field family. CheckboxField / SwitchField are unrelated chrome and are not touched.

PR Checklist

• Added tests for bugs / new features (BorderedTextField has its own suite; existing TextField/SelectField/TextArea tests pruned of removed behaviors)
• Updated docs (BorderedTextField stories added; TextField/SelectField/TextArea stories updated to remove bordered examples and endSlotPosition)
• Reviewed and approved Chromatic visual regression tests in CI