fix: discrete log color sampling: build continuous log CTF before samp…

[patrickoleary]

Fix discrete log color sampling - Change sampling as we did with symlog

• Build a continuous log CTF (256 samples uniform in log space) before sampling discrete band colors, matching the pattern used in the symlog fix
• Discrete bands now sample from this log CTF instead of directly from the linear CTF with manual position mapping, ensuring colorbar and 3D rendering colors match
• Make eps_lin relative to data_range to avoid overlap/bleed issues at extreme data scales

Test small numbers and big numbers

closes #74