stb: generate a library theme from a single seed color (scheme math + dark derivation)

Summary

Add a generator to the stb theming tool that builds a full paired light/dark theme from one seed brand color, instead of hand-authoring every token. Composes two pieces of well-known OKLCH color math.

How it works

1. Seed → harmonious light palette (pure hue rotation in OKLCH, keep L/C):

• Complementary: H + 180
• Analogous: H ± 30
• Triadic: H ± 120
• Split-complementary: H + 150, H + 210

Pick a scheme, rotate the seed hue to fill primary + accent/secondary tokens.

2. Light → dark via perceptual lightness inversion: L → 1−L, C × 0.5, small hue shift. (Same derivation as the stb peer-dark-model work.)

Result: one seed color → a complete, coherent light + dark theme, addable to stb's built-in preset library.

Why

• stb presets (loadBuiltInPreset + preset menu) are hand-authored today — slow and inconsistent. A generator makes new library themes cheap and coherent by construction.
• Lowers the barrier for contributors/operators to brand Stratos: choose a brand color, get a usable theme.

Dependencies / relationships

• Requires OKLCH on the token path — rides #5515 (RGB→OKLCH migration).
• Reuses the light→dark derivation helper from the stb peer-dark-model + dual-field-editor work.
• Enhancement / future work — not blocking current stb facet work.

Related: #5515 (RGB→OKLCH), theming/branding (stb) workstream.

Start by reviewing stb's loadBuiltInPreset and preset menu, then inspect the OKLCH migration in #5515 and the peer-dark-model and dual-field-editor work for the existing light-to-dark derivation. Done means a seed-color generator produces paired light/dark themes and integrates them into stb's built-in preset library.