feat: write each poster label in its paired M3 role, as Material Theme Builder does

[abernier]

What

Each label of the Scheme / Shades poster is now written in the M3 role it is paired with, the way the official Material Theme Builder poster paints them, instead of white in mix-blend-mode: difference:

• primary reads in on-primary, on-primary in primary, and so on for every * / on-* and *-container / on-*-container pair (primary, secondary, tertiary, error, custom colors);
• the surface family (surface-dim ... surface-container-highest, outline-variant) in on-surface; on-surface and outline in surface; on-surface-variant in surface-variant;
• inverse-surface / inverse-on-surface swapped, inverse-primary in on-surface;
• scrim and shadow, black in both schemes, in plain white;
• tones 80 and up in black, the rest in white.

The fixed accents, which the official poster does not draw, follow the same pairing (primary-fixed* in on-primary-fixed, on-primary-fixed* in primary-fixed), as do the dropped roles (background / on-background, surface-variant in on-surface-variant, surface-tint in on-primary).

The Tailwind story's hand-written tiles follow suit: text-on-myCustomColor1 & co. for the custom colors, black on shades 50 to 200 (tones 95 to 80) and white below.

In tw mode the ink stays a var(--md-sys-color-*): that mode is about the bg-* utilities resolving.

How the pairing was read

The official app is a Flutter canvas, so its poster was pixel-sampled: tile backgrounds segmented, each label's glyph color matched hex-equal against the other tiles' backgrounds in the same scheme, on both the light and dark scheme and the six tonal palette rows.

Checks

• pnpm run lgtm green (318 tests).
• Storybook checked by computed style: every label's color equals its paired role's background, on light, dark, custom colors and tones.
• Chromatic will flag every poster story: expected, every label changes color.

Pre-existing, out of scope: in the Tailwind story the Secondary tile renders light (bg-secondary collides with shadcn's theme).

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