Add an explicit failure path for color pairings that fail contrast

[codeswithroh]

What this changes

Closes the remaining gap in #8. Semantic roles with attached ratios (the Color contract in style-lock-format.md) and use-time re-checking of invented pairings (SKILL.md's non-negotiable #5) already shipped in earlier work. Missing was the third piece reviewers explicitly flagged: what happens when a pairing genuinely fails. Without a defined path, "iterate until it passes" has no way to distinguish a real fix from looping on the same failing value.

• SKILL.md — non-negotiable Motion defaults assume a scroll-driven marketing page #5 gains an ordered failure path: reuse an already-legal pairing → nudge the new color's lightness within its own hue family and re-verify with the matrix (the same approach generate_palette.py already uses at generation time, applied ad hoc to one token) → fall back to a known-safe neutral if nudging would break the color's intended role → surface a genuine conflict to the user rather than shipping the failure or silently substituting something unasked for.
• references/style-lock-format.md — Color contract section points at the failure path and adds a note field for recording when a token's value was nudged mid-build for contrast.

Closes #8

Test plan

• Verified fence balance and no stray "preset" mentions in both edited files
• Cross-checked the failure path doesn't conflict with existing generate_palette.py behavior — it's the same lightness-solving principle applied to a single ad hoc token instead of a full palette