[feat] drag to place from the Explorer (roadmap 39 P1-P6)

[AlexZ005]

Roadmap 39 (lane 39-place): drag to place from the Explorer, P1-P6 (VR radial placement deferred to a follow-up).

What it does

• Drag a pack item / library model / prefab from the Explorer onto the viewport: a ghost shows where it lands
  (resting ON the surface under the pointer, grid snap unless Shift) and how big it is (W × D × H in metres +
  tris + download size). R / Shift+R / the wheel turn it 15°; Alt = the camera's focus point (dropped
  onto the surface below it); snapTargets.alignNormal tilts it onto the surface; a multi-selection drops as a row.
• Release over the viewport = spawn at the ghost: one undo step, replicated like any add. A static pack item
  spawns as a pack stub (30c reference) drawn in the Placeholder Style until it loads; every peer refills it.
  Animated items, library models and prefabs take their import paths at the ghost pose.
• Esc, releasing anywhere else, or dropping back on the Explorer = nothing spawns, no undo entry.
• Ghost tiers: real model (shared geometry, one translucent material, no shadows) when decoded and allowed by
  Settings ▸ Scene ▸ Performance ▸ Placement preview (Full / Within budget (default, 50k; 15k phone/Quest) / Box
  only); otherwise the box. Hovering the viewport prefetches. Amber = mostly inside another object (still places),
  red = no surface (release does nothing).
• Touch: long-press a card, the Explorer tucks away, drag, lift to place.
• Pack downloads: a CacheStorage cache (packCache.js) — Downloaded badge + size on pack cards, a
  Downloaded filter in the Packs, item menu Delete cache (size) (the item stays in the pack), pack row
  Delete downloads, Storage Pack downloads + Clear all pack downloads.
• Pack rows carry size [w,h,d] / box / tris / bytes / animated (packs PR); missing → measured on first
  load and kept in idb; library models record their dims at import. PACKS.md documented.

Design note: a pointer drag with an HTML5 bridge

A native drag delivers no keydown/wheel, so R/wheel/Esc cannot work on it. A trusted dragstart on a placeable card
is cancelled and placeDrag.js runs the gesture; over any non-viewport element it re-dispatches the card's
application/x-explorer-item payload as synthetic dragenter/over/leave/drop (+ dragend on the source), flagged
application/x-tp-place-bridge, which App's window drop ignores. Every existing HTML5 target keeps working
(Explorer folders/packs/bin, Inspector texture slot, LOD panel, pickers). Synthetic dragstarts (suites) keep the
native path.

Proof

• place-drag 39 (two peers, real mouse): row dims, model swap, rotation, Esc / back-to-Explorer = no spawn + no
  undo, one-undo drop at the exact pose, stub replicated to B and refilled there, undo reaches B, release over chrome
  places nothing (counterfactual: 0 → 1 with the App guard removed), bridge moves a card into a folder.
• place-drag-cache 33: downloads, badge, filter, Delete cache (bytes gone, item stays), Storage clear-all, preview
  tiers, the Settings rows, touch long-press at 390×844 (counterfactual: without the touchmove block every
  finger drag ended in pointercancel).
• place-drag-extras 15: multi-select row as one undo, Alt focus on the ground, red over the sky.
• place-drag-perf (exclusive): a cached 200,340-tri item with the default budget → box ghost, frame time
  16.67 ms dragging vs 16.67 ms idle, no long tasks (headless rAF is vsync-capped).
• vitest +13 (1885 total), svelte-check 300/41 (= baseline), check:tokens clean.
• Regression battery: see the lane handover for the A/B table.

Shots: ~/.code/lanes-30/after-39/39-place/NN-*-{before,after}.png; docs fragment beside them.

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