Tube Builder Lab: per-tube stabilized crop window + comparison viz

[Chouffe]

Summary

Adds per-tube crop stabilization to the Tube Builder Lab so the temporal head can be shown a static crop window with the smoke moving inside it, instead of the current jumpy per-frame crop that recenters/rescales every frame.

• stabilize.tube_window(tube) — pure function returning the union (enclosing) box of a built tube's observed detections, normalized. Does not mutate the Tube (stabilization is a crop-window decision, not a tube-building step). No separate margin: the crop step supplies context via CROP_CONTEXT.
• viz.crop_box_px — single source of truth for the crop region (expand by CROP_CONTEXT, square). Used by both the crop and the drawn box, so the box you see equals the crop you get. crop_tube_at_frame now routes through it and serves both per-frame boxes and union windows; draw_tube_windows draws the same region in the tube colour.
• app.py — removed the former (current) builder from the app (the candidate is now the only baseline); added a non-stabilized vs stabilized comparison: a two-up main viewer (left per-frame box, right fixed union window, tube-coloured, active-only) and a per-tube crops strip in the "candidate crops @ this frame" expander.
• CROP_CONTEXT lowered to 1.6 (single shared knob, tightened by eye below the model's training context_factor ≈ 2.0).

Scope: lab-only (approach A). Nothing in bbox_tube_temporal changes. Lifting tube_window into the lib crop path (approach B) and retraining on the new crop distribution are out of scope — see the spec.

Spec: experiments/temporal-models/tube-builder-lab/docs/specs/2026-06-05-stable-tube-crop-window-design.md

Test Plan

• make lint — clean
• make test — 80 passed (tube_window: union/single/axis-independent/gaps/empty-raises; crop_box_px: square+context-scaled+centred, edge clamping; existing regression snapshots still pass)
• make app — visually confirm the stabilized window stays fixed while the per-frame box moves, on a growing-plume target (e.g. platform_43096)