docs(research): depth fade vs deep catalogs (Milliquas) investigation

[rulkens]

Summary

Doc-only. Adds an investigation memo diagnosing why the default-on Depth fade toggle erases almost all Milliquas quasars past z ≈ 0.2, and maps the fix onto the points pipeline. No code changes — the memo is written to feed directly into a follow-up implementation plan under docs/superpowers/plans/.

New file: docs/superpowers/research/2026-05-21-depth-fade-deep-catalog.md

What the memo covers

• Diagnosis. Depth fade is 1/(1+(camDist/1000Mpc)²) (vertex.wesl:184-188) — a galaxy-density anti-saturation hack tuned to the GLADE volume. With the camera near the origin it acts as "attenuate by redshift", so by z ≈ 0.5 quasars sit at ~18% brightness and by z ≈ 1 at ~5%. A hard invisibility cull (vertex.wesl:224, intensity < 0.005) means deep quasars past z ≈ 1.5–2 are not just invisible but non-pickable — the name sidecar / InfoCard can never fire for them.
• Two distinct defects: D1 — wrong scale for a sparse, deep catalog; D2 — camera-absolute fixed scale dims the whole field (galaxies included) once you zoom out to frame deep objects.
• Four ranked options, each with a code sketch and exact file/line touchpoints, no committed direction:
  • A — per-source falloff scale
  • B — skip the fade for sparse (quasar) sources
  • C-lite — focal-relative adaptive scale (one float into the free _pad4 slot; galaxy view unchanged via a max(1000, focalDist) floor)
  • C-full — true view-axis focal plane (needs a vec3 → struct resize)
• Plumbing facts that constrain any fix: byte-matched uniform layout across io.wesl / pointRenderer / pickRenderer, the one free 4-byte slot at offset 124, automatic pick parity via the shared uniform buffer, and cam.distance as the focal-distance source.

No on-disk .bin format impact; all options touch only runtime uniforms + the shader.

Test plan

• Read the memo end-to-end and decide which option(s) to pursue (A/B/C-lite/C-full compose; they don't exclude)
• Confirm the §3.1 redshift→fade numbers match the intended diagnosis
• When a direction is chosen, spawn a TDD plan under docs/superpowers/plans/ (the §5 options are written to feed directly into that)

https://claude.ai/code/session_01H9ifaFejATHQ2qWcPVTUrn

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