Expand render.Styles into a complete token set (component slots + Glyphs)

[joestump-agent]

Part of #54. This is the foundation issue — the terminal analog of the web basic catalog's --a2ui-* CSS variables.

Problem

render.Styles has six fields (CardBorder, Heading, Subheading, Caption, Button, ButtonFocused), so a host can theme those — but most chrome is hardcoded literals a host cannot touch:

• focus cues ▏/▎ (render/fields.go:70-73, :104, :145-149, :196, :228)
• button brackets "[ "/" ]" (render/button.go:38)
• checkbox/radio marks [x]/[ ]/(•)/( ) (render/fields.go:89-91, :130-137)
• list bullet "• " + continuation indent (render/containers.go:175-185)
• tab separator " │ " (render/containers.go:120), modal cues ▹/▸ (:136-139)
• divider glyphs ─/│ (render/styles.go:69, containers.go:87), slider cells █/─ (fields.go:176, :186)
• several roles share one slot: input labels, placeholders, (empty)/(unset), {binding} diagnostics, and media placeholders all reuse Caption

Proposal

Extend Styles with per-component slots and a Glyphs struct:

type Styles struct {
    // existing
    CardBorder, Heading, Subheading, Caption lipgloss.Style
    Button, ButtonFocused                    lipgloss.Style

    // new component slots
    ButtonPrimary, ButtonPrimaryFocused lipgloss.Style // see the primary-variant issue
    Label, Placeholder                  lipgloss.Style // default: alias Caption
    InputText, InputFocused             lipgloss.Style // default: unstyled
    Divider, ListBullet                 lipgloss.Style // default: unstyled
    TabActive, TabInactive              lipgloss.Style // default: Heading / unstyled
    SliderFilled, SliderEmpty           lipgloss.Style // default: unstyled

    Glyphs Glyphs
}

type Glyphs struct {
    FocusCue, FocusCueActive  string // "▏", "▎"
    ButtonOpen, ButtonClose   string // "[ ", " ]"
    CheckOn, CheckOff         string // "[x]", "[ ]"
    RadioOn, RadioOff         string // "(•)", "( )"
    ListBullet                string // "• "
    TabSeparator              string // " │ "
    ModalCue, ModalCueOpen    string // "▹", "▸"
    DividerH, DividerV        string // "─", "│"
    SliderFilled, SliderEmpty string // "█", "─"
}

Rules

• Byte-compatible defaults: DefaultStyles() fills every slot with today's hardcoded values; all existing goldens and the no-color-SGR test (render/styles_test.go:17-37) pass unchanged.
• Alias at construction, not render: defaults that alias an existing slot (Label = Caption) are set inside DefaultStyles(), so overriding Caption alone keeps today's behavior while a host can split the roles.
• Zero-value glyph fallback: NewSurface backfills empty glyph strings with defaults so a partially-populated Styles from an older host doesn't render blank chrome.
• Renderers switch from literals to s.styles.* / s.styles.Glyphs.* — no behavior change with defaults.

Out of scope

• honoring new hints (separate issues)
• Surface.SetStyles (separate issue)
• createSurface.theme mapping — stays a no-op per the epic's invariants