docs(backlog): reframe Milliquas colour item as AGN-colormap follow-up

docs/BACKLOG.md

@@ -63,7 +63,7 @@ Diagnosed but unplanned. Captured here so they don't get lost; promote to a spec
 - **Mobile layout reflow** — hover-on-touch is handled (`disable hover on touch input`, #226: hover-only affordances now route through tap). What remains is the general responsive layout pass: reflow the InfoCard / SettingsPanel / StatusBar for narrow viewports so the UI is usable on a phone, not just non-broken.
 - **Lower-tier "close to home" weighting** — retune the small/medium tier subsampling so more galaxies survive near the camera's home position for maximum visual density on first load, while keeping the on-screen count fast. Distinct from the deliberate SDSS far-shell sample (memory `project_sdss_medium_intentionally_far`).
 - **Densely seed the Local Volume across all tiers (group explorability)** — surfaced 2026-06-04 with the `group` category. The 16 Local Volume groups are only interesting to fly into if their *member* galaxies are present, but `subsampleByAbsMag` (`tools/catalog/`) thins the nearby volume by absolute-magnitude cut, so faint dwarfs in the Local Group / M81 / Cen A / Sculptor etc. get culled — a group ring you focus into can be nearly empty at small/medium tier. Bias the subsampling to **keep galaxies inside (or near) the featured group spheres** regardless of `M_abs`, across small + medium and ideally large tiers, so each group has as many members as possible. Related to but distinct from the "close to home" weighting above: that's camera-home density; this is per-group membership density keyed off the structure seed. Implementation hooks: the group seed positions/radii (`data/structure_anchors.seed.json`) are available to the build, so the subsampler can spare points within `apparentRadiusMpc` of each group centre. Keep an eye on the on-screen count budget. Pairs with the cluster-focus member count (`PoiDetailCard` "Galaxies" row) — denser seeding makes that number meaningful at lower tiers.
-- **Milliquas colour check** — Milliquas points currently all render blue; verify the colour-index / colour mapping for the quasar source isn't collapsing to a single hue.
+- **Milliquas needs its own colormap (AGN ≠ galaxy)** — Milliquas points render overwhelmingly blue. The *clamp* half shipped (#282): the redshift K-correction (`kPerZ`) was subtracting more than the whole `[0,2]` ramp span for high-z quasars and pinning every row to the blue floor; `kPerZ` is now 0 so the real B−R spread survives. What remains is the **semantic mismatch**: quasars genuinely have small B−R, so on the galaxy star-forming↔elliptical ramp they legitimately land in the blue third, but "blue" there means "star-forming galaxy" — the wrong reading for a non-thermal AGN continuum. The fix is to give AGN their own visual encoding instead of reusing the galaxy ramp. Directions to weigh in the brainstorm: (a) a distinct AGN ramp (violet/amber) keyed on B−R so quasars read as a different object class; (b) encode **redshift** instead of colour (z is the meaningful axis for objects spanning the observable universe); (c) tint by the Milliquas **class byte** (Q/A/B/K/N/S) or parent-survey byte — both already on the `.bin`. Likely needs a `colourMode` discriminant on `SOURCE_REGISTRY` + a shader ramp branch; brainstorm → spec → plan, not a drop-in.
 - **Tour feature (full)** — finish the camera tour beyond the Part-2 seed. Tracked design: [splash-screen Part 2 plan](superpowers/plans/2026-05-20-splash-screen-02-stub-tour.md) (ready to execute; engine-side `engine.tour` seed, verified current 2026-06-04) and [2026-05-07 tour-animation spec](superpowers/specs/2026-05-07-tour-animation-design.md) (re-grounded 2026-06-04; labels + MW impostor + engine-API decisions resolved, cinematic palette documented). Execute Part 2 first, then resolve the spec's remaining open decisions (rotation slerp, caption producer, beat list/timing), promote to a plan, and extend the seed into the real waypoint tour.
 - **Thumbnail quality (SDSS / DSS branches)** — the auto-fetched SDSS-cutout and CDS-DSS thumbnails still have the original quality issues: ranked fix options are mask, sky-sub, per-galaxy size, DESI source, brightness norm (see memory `project_thumbnail_quality`). The *famous-galaxy* branch is now fully addressed — procedural-disk fade-out, high-res LOD (#214), and thumbnail calibration + square deproject + disk-plane unification (#229/#234/#235/#240) all shipped — so this item is scoped to the non-curated SDSS/DSS path only.
 - **Supercluster/wall shape accuracy (focus mode)** — cluster-focus mode (PR #242) renders membership as a sphere of radius `apparentRadiusMpc ?? physicalRadiusMpc` centred on the catalog centroid. For superclusters/walls (MSCC) this is crude: the structure is a flattened sheet, so the sphere swallows foreground/background voids and clips the wall's arms (e.g. Hydra Wall reads ~847 galaxies at medium tier). No all-sky per-galaxy membership catalog exists to replace it — redMaPPer/WHL give cluster member galaxies but only in the SDSS footprint; Liivamägi+2012 gives galaxy→supercluster IDs but is also SDSS-limited and threshold-dependent. Investigate a better proxy: (a) **ellipsoid fit** from MSCC member-cluster positions (`memCl` column — data we already have); (b) **density-field membership** reusing the rhizome/MCPM cosmic-web field or DisPerSE filaments (all-sky, same method the literature uses). Option (a) is cheap and immediate; (b) is more principled and reuses existing plumbing.