[snkmcb]

Summary

This is the first slice of the proposed typed drag and drop: moving one item within a single ListView or TreeView.

• Public API. Adds CollectionDragDispatcher and collection_drop_target in collection_drag.hpp. No new document vocabulary, roles or semantic state.
• Clicks still work. The dispatcher observes a row press without consuming it, so click selection and focus on press behave as before.
• Drag behavior. After a 4-unit threshold (configurable), the result reports started and the host resets the ordinary pointer press. Moves report the source key and a before/after insertion point at the nearest displayed row. The release reports it as drop; releasing outside the collection cancels.
• Cancellation. Escape, Cancel and PointerCancel cancel and are consumed. Other key, text and logical input cancels unconsumed.
• Replacement. refresh recomputes the target at the last position, so a wheel scroll moves the insertion point. It cancels when the collection or the dragged row disappears, collapses away or becomes disabled.
• Inspector: typed drop command. Scene's children are reordered through a node-free item.move command (item/target object IDs, placement enumeration), resolved with CommandSnapshot::resolve over published item IDs. Moving Scene, dropping before Scene and no-op drops reject with item_move. Item data, selection and focus are kept.
• Inspector: feedback and keyboard. The dragged row is dimmed to 45% opacity and a valid target shows a two-unit blue inset band on its top or bottom edge. Alt+Up/Down is the keyboard equivalent. A help line describes both.
• Docs. New input architecture section (the proposal section now names what remains), TreeView/ListView guide section, Inspector section, support matrix, testing table, backlog and changelog.

Verification

• New owner test collection_drag: insertion points (halves, gaps, below rows, clipping, collapsed and disabled), threshold, click pass-through, toggles, drag/drop, cancellation paths, scroll and replacement refresh, rejection.
• inspector_hierarchy window-free smoke: new reorder_smoke. The injected native smoke drags Light below Camera with Windows mouse messages under OS capture.
• Debug: all 98 non-native tests pass.
• Release: collection_drag, inspector_hierarchy and inspector_hierarchy_native pass.
• scripts/check-docs.ps1 passes. Formatted with clang-format 23.1.1.

This PR is independent of #196 (number scrubbing). Both edit examples/inspector-hierarchy/main.cpp, CHANGELOG.md and the Inspector guide, so whichever merges second needs a small rebase.

Not included

Payload types beyond item keys, copy/link operations, drag previews, dragging between collections or into other targets, external OS drags, automatic scrolling while dragging, reparenting, transactions/history and native accessibility.

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