Give Button `variant: "primary"` a visual identity via ButtonPrimary slots

[joestump-agent]

Part of #54. Depends on the token-set expansion (#56).

Problem

renderButton (render/button.go:39-41) acts only on borderless; primary is explicitly ignored with the documented rationale (render/button.go:14-20) that monochrome leaves no free attribute — bold is the base style and reverse-video is reserved for focus.

That rationale is correct for the default styles but dissolves once hosts can inject color: a themed host (crush mapping its accent color) has an obvious way to distinguish the main call-to-action, and the A2UI basic catalog explicitly defines primary as "the main call-to-action button".

Fix

• Route Variant == "primary" through the new ButtonPrimary / ButtonPrimaryFocused slots from Expand render.Styles into a complete token set (component slots + Glyphs) #56.
• DefaultStyles() sets ButtonPrimary = Button and ButtonPrimaryFocused = ButtonFocused, so monochrome output is byte-identical to today — the distinction only appears when a host overrides the slots.
• Update the rationale comment in render/button.go to describe the new mechanism.
• Test: default render unchanged for a primary button; with an overridden ButtonPrimary, primary and default buttons render differently and focus still uses the focused variant.