fix(ios): give a filled button a visible edge when its tint matches the surface

[CodyPChristian]

Problem

A filled button whose tint resolves to the same colour as the surface it sits on has no visible edge — it reads as text, not a control.

This is easy to hit with a themed palette. A site whose brand colour is a pale cream and whose card surface is that same cream renders its primary action invisible:

primary: #F4F1EC   surface: #F4F1EC   →  "Sign in" vanishes into its card

Fix

When primary or secondary collides with the surface, substitute a token that contrasts and keep the button filled. Filled matters — outlining preserves the brand colour but reads as a secondary control, which is wrong for a screen's main action.

The two-parent guard

The renderer cannot see what it is actually sitting on. NodeView, which paints container backgrounds, lives in the host package, so no ambient-surface value reaches the button — and the same button may be used on a card (surface) or directly on the page (background).

So a candidate is only accepted when it contrasts with both. Without that guard, a palette whose secondary equals its page background gets fixed on the card and goes invisible on the canvas:

secondary #314E5E  vs surface #F4F1EC  ✓ contrasts
secondary #314E5E  vs background #314E5E  ✗ identical  → rejected
on-surface #0F172A vs both  ✓  → accepted

on-surface is tried next and contrasts with the surface by construction.

Scope

• Semantic variants (destructive, success, accent) are never substituted — a slate Delete button would not read as destructive. They fall back to a hairline outline in their own label colour, as does any variant with no qualifying candidate.
• Detection is a channel-delta threshold (~0.12) tuned to catch "same colour to the eye" without firing on merely light-on-light, so a normal palette is untouched.

Verified against four production white-label palettes: only the colliding one changes behaviour, the other three resolve identically to before.