Add qoolqit.utils.colors with the Qoolqit palette and colormaps

[alessandro-santini]

Description

Adds qoolqit/utils/colors.py, defining the qoolqit palette (9 colors) and
13 colormaps, and registering them with Matplotlib by name.

Purely additive: the import registers new names but never touches rcParams, so
existing plots are unaffected, no Matplotlib built-in is overwritten.

Colors apply only where they are asked for explicitly.

Usage

from qoolqit.utils import colors

plt.plot(x, y, color="mint_green")
plt.imshow(data, cmap="purple_mint")
plt.imshow(signed, cmap="orange_mint_dark", vmin=-v, vmax=v)

What's registered

• Colors: metal_blue, neon_purple, mint_green, soft_orange,
  dark_green, neon_blue, soft_green, bright_green, neutral_gray.

• Colormaps: <low>_<high> diverging (purple_mint, orange_mint,
  purple_orange, blue_mint, each with a _dark near-black-center variant)
  and <hue> sequential (mint, purple, orange, deep, night), plus
  every _r reverse — 26 in total.

  Palette

Name	Hex		Name	Hex
dark_green	#0F1E23		neon_blue	#92C8E5
soft_green	#173035		neon_purple	#867BFA
bright_green	#E1F6E9		soft_orange	#FF986E
mint_green	#00C887		neutral_gray	#506166
metal_blue	#397378

[github-actions]

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