Extend edit controls to Bdv/Bvv

[tferr]

This is major refactoring: the node-editing logic of the ImagePlus tracing canvas now lives in a viewer-agnostic class, so the same actions of the ImagePlus contextual menu are available in Bdv & Bvv (context menu, single-key shortcuts, hover-based active node, undo, etc.).

New classes

• PathNodeEditor: Operations on a single Path (delete, insert, move, lock, radius, tag, re-root, split, connect, undo). Returns a Result that front-ends route with Result#dispatch(). Shared helpers: deleteNodeOrPath(gui), tagNode(path, index, color), replaceTree(), structuralChange()
• NodeEditCommand / ContextMenuCommand: Host labels, icons, default shortcuts and menu population, shared by the canvas and the viewers
• BigViewerEditor / BigViewerMenu: Bdv/Bvv interaction layer (hover picking, key bindings, right-click menu) on top of PathNodeEditor

SNT / SNTUI changes:

• SNT holds the PathNodeEditor (editing path, active node, undo history) shared by canvas, Bdv and Bvv; adds enterEditMode(Path), exitEditMode(), getEditModeBlocker(), notifyActiveNodeChanged() and notifyEditingPathChanged() (throttled)
• enterEditMode(Path) selects the path first, as all edit operations require the editing path to be selected
• editModeAllowed() no longer swaps the editing path while already editing; Stream mode restores its tracing state on exit
• SNTUI fans Edit Mode notifications out to open viewers only (getOpenBigViewers() skips disposed ones) and tracks the most recently activated viewer

Fixes / Behavior changes:

• Bvv AnnRenderer: screen data and layer ranges are published as one immutable Frame/Source pair, fixing the unsynchronized screenData race (NPE in tests)
• Bvv#getClickedPosition(null) no longer returns a stale cached position
• SNT#startSholl() converts calibrated coordinates to pixel indices
• Sholl at nearest node in viewers: selected paths, else the rendered path nearest to the cursor (same lookup as Grab Nearest Path)
• Bookmarks added from a viewer in classic mode use the viewer's current timepoint; BookmarkManager moves open viewers and image windows, and reports locations outside the (materialized crop of the) image
• Radius prompt: tolerate a corrupted saved value, message says "non-negative" (0 is accepted)
• Deleting a whole path from the canvas now leaves Edit Mode; refusing to delete a junction node opens a dialog
• Right-clicks no longer trigger tracing clicks in the viewers; M adds a bookmark in classic mode (it was blocked)
• Removed the "Continue Extending Path" toolbar toggle from Bvv/Bdv (the action remains in the context menu)
• Misc fixes