feat(theme): make the neutral border tokens opaque surface steps

[gabriel-lisboa-azion]

Summary

• --border-default and --border-muted become opaque steps of the surface palette. As alpha they painted a different colour on every backdrop — rgb(26) on --bg-canvas, 35 on --bg-surface, 44 on --bg-surface-raised, 95 on --bg-surface-overlay — and two rules meeting composited into a fifth, brighter value, so any stacked or abutting border read as a double rule.
• Adds four palette steps (gray 150 / 250 / 750 / 775) because the ramp had none in the range a border needs: it jumped 200 (229) → 300 (178) and 700 (77) → 800 (26), and those bracketing steps are already --bg-selected / --bg-disabled, so an opaque border landed on its own fill and disappeared. The dark background anchors (700 / 800 / 900) keep their values.
• muted now sits closer to the background than default in both themes — lighter in light, darker in dark — instead of being identical to it in light.

How to test

1. pnpm storybook:dev → Components/Inputs/Checkbox → Disabled, both themes: the control keeps a visible border. Before this change an opaque border here was exactly --bg-disabled and vanished.
2. Any bordered component on a raised or selected surface (Components/Data/Table, Components/Overlay/Panel, Components/Actions/SegmentedButton): every hairline is visible, and a rule reads the same weight wherever it is drawn.
3. pnpm --filter @aziontech/theme test → 12 pass.
4. node packages/theme/src/scripts/build-tokens.mjs → dist/v4/globals.css regenerates with no diff, confirming the committed dist matches source.

Notes

• Not an API change: no token is renamed or removed, only revalued, plus four additive palette steps. It is a visible change to every bordered component — borders read slightly stronger and no longer drift by backdrop — so it wants a design eye as much as a code one.
• Measured contrast against every surface role, both themes: minimum delta 10, no zeros. Previously four combinations were delta 0 (invisible) once opaque.
• Blocks feat(webkit): add frame-box, section-gap, section-title and hero-title #880, which needs these tokens for its frame rules and hatch to reach full strength.