Chart: revive target cross, add nearby-DSO markers + zoom-scaled mag filter

[brickbots]

Summary

The chart screen's target cross was dead code — ui_state.set_target() lost all callers in the object_details/object_list refactor, so ui_state.target() was always None and the cross never drew. Nearby catalog DSOs were never plotted (only a loaded observing list ever was), so on a fresh session with no list loaded the chart looked bare. This PR fixes both and adds a zoom-scaled magnitude filter for the new nearby layer.

What changed

• ui/object_details.py — set_target(self.object) in update_object_info(), the single chokepoint for open / scroll / eyepiece-cycle / display-mode-cycle. The cross now tracks the last-viewed object immediately (not push-to, no 10s dwell lag).
• ui/chart.py — plot_markers() restructured into three deduped layers:
  • Target cross — its own pass, full brightness, independent of chart_dso (survives chart_dso=0), with its designator label when on-screen (off-screen shows the existing pointer arrow, no label).
  • Observing list (unchanged) + new nearby layer, both at chart_dso brightness.
  • Deduped by object_id, precedence target → observing-list → nearby.
  • New module-level dso_mag_limit(fov): linear mag 11 @ 5° → mag 7 @ 60°, clamped; unknown mags (filter_mag == 99) hidden.
  • Nearby set = active "All Filtered" catalog objects within fov * 0.75°, mag-filtered, capped at the brightest 20. The BallTree is rebuilt only when the filter's dirty_time changes or deferred catalogs finish loading — never per frame; the radius query runs on the new-solve path only.
• nearby.py — ClosestObjectsFinder.get_objects_within_radius() via BallTree.query_radius (haversine), reusing the existing M/NGC dedup.
• main.py — mark the filter dirty on catalogs_fully_loaded so the chart's index rebuilds to include newly loaded (e.g. WDS) objects.
• docs/ax/ui/CONTEXT.md — new Target glossary term resolving target vs selected object.

Mag-curve endpoints and the fov * 0.75 radius margin are starting values to tune on-device (per the plan).

Testing

• Unit — dso_mag_limit endpoints/clamp/linearity; get_objects_within_radius correctness + dedup; nearby selection mag-filter/cap/type-filter, dedup precedence, and index-rebuild-only-on-dirty (stubbed Starfield, no hip_main.dat needed).
• Integration — set_target tracks the object on open and scroll via a real UIObjectDetails; existing chart UI-module smoke still passes (real Starfield + real solve).
• Live (headless app) — verified the on-screen cross + "M 57" label (Ring Nebula on-field), the off-screen pointer for NGC 1, and nearby markers rendering.

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