feat(cycle): Implement scheme rings

[bezhermoso]

Addresses #135.

Summary

This adds configurable cycle rings for tinty cycle. Instead of relying on a single preferred-schemes list or inferring light/dark relationships from scheme names, users can now define explicit named groups of schemes and cycle through the group they want.

The issue discussion called out that suffix-based light/dark inference is unreliable because scheme names are conventions, not guaranteed metadata. Explicit rings avoid that ambiguity and also support arbitrary user-defined cycles beyond light/dark groupings.

Configuration

default-cycle-ring = "default"

[[rings]]
name = "dark"
schemes = ["base16-gruvbox-hard-dark", "base16-ayu-dark"]

[[rings]]
name = "light"
schemes = ["base16-gruvbox-medium-light", "base16-github"]

[[rings]]
name = "default"
schemes = ["base16-gruvbox-hard-dark", "base16-github"]

• tinty cycle uses default-cycle-ring.

• tinty cycle --ring dark cycles through the named ring directly.

• Empty selected rings now return an error instead of applying anything.

preferred-schemes

preferred-schemes is no longer used. If it is present, tinty cycle exits with a hard migration message and prints a copy/paste [[rings]] replacement that preserves the old cycle order, including default-scheme prepending behavior.