feat(theme-studio): color-families export round-trip and README close-out

Export stays a flat palette and import needs no reconstruction, because families are derived from the hex every render rather than stored. A #roundtriptest gate confirms export to import to export is byte-identical, and that the exported palette is still a flat [hex, name] list. Package seeding is unaffected since it reads the same flat palette.

The spec's planned ramp-step warning exemption is dropped after analysis: a generated ramp's steps are a stepL apart, well above the too-similar ΔE threshold, so they never trigger the warning, and exempting same-family pairs would hide genuine near-duplicates that should be flagged (the case #deltatest checks). So the warning stays on the full palette.

README documents color families: the hue grouping and its limitation, the ground strip, the per-column count control and regenerate, removed-step references reading "(gone)", and the removal of the standalone ramp panel. Phase 6, the last phase; the color-families v1 build is code-complete.

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