Circuit-Editor Multi-Target Gate Support

[ScottCarda-MS]

Circuit Editor: multi-target / group drag-editing

Reviewer TL;DR. This adds the multi-target / group drag-editing layer
to the circuit-vis circuit editor: moving multi-target gates and groups as
rigid units, cloning them as a unit (Ctrl-drag), and shift-to-extend a group
onto new wires. No public API changes. The diff is small and
self-contained — 8 files — and the behavior is purely additive on top of the
existing single-target editing paths.

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At a glance

Scope	8 files — 4 source, 4 test — all under source/npm/qsharp/
Size	+1,439 / −23
Language	TypeScript (frontend) + .mjs tests. No Rust, no VS Code host.
Public API	Unchanged
Tests	+26 tests, all green
Behavioral risk	Additive — only the multi-leg editing paths are new

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What this PR adds

The multi-target / group drag-editing behavior, layered onto the
single-target editing foundation:

1. Rigid unit-move. Dragging a multi-target gate or a group shifts the
   gate's own .targets and recursively remaps every register reference in
   its child grid by the same delta, so the box and its contents stay aligned.
   Classical controls anchor correctly:

   • an external measurement's classical reg stays put → the moved gate's
     classical control anchors on the original wire;
   • an internal measurement (inside the moved subtree) moves with it → the
     consumer's classical control shifts in lockstep, and numResults
     bookkeeping follows the measurement.
   • Bounds-checked: a unit-shift that would push any wire below 0 is refused
     without mutating the model.

2. Clone-as-unit (Ctrl-drag). Ctrl-dragging a multi-target gate / group
   leaves the source in place and drops a full copy at the target, with the same
   child-register remap as a unit-move.

3. Shift-to-extend. Holding Shift while dragging a child out of an expanded
   group offers dropzones on wires outside the group's span (skipping wires
   blocked by ancestor-column siblings) and paints a ghost border showing the
   extended bounds — extending the group onto new wires.

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Files changed

Source (4):

File	What it adds
actions/circuit-actions/move.ts	moveAsUnit, shiftAllRegisters, subtree wire-swap helpers; the unit-shift branch of moveY; group-body target handling.
actions/circuitActions.ts	moveOperation unit-shift wire-growth; addOperation clone-as-unit + sourceWire param.
editor/controllers/dragController.ts	Shift-extend lifecycle (setupShiftExtend / spawn / paintGhostBorder / tearDown) + the per-op multi-leg dropzone loop.
editor/draggable.ts	makeShiftExtendGhost geometry helper.

Tests (4):

File	Tests added
test/circuit-editor/circuit-actions/groupClone.test.mjs	7 clone-as-unit tests (whole file).
test/circuit-editor/circuit-actions/groupMove.test.mjs	7 rigid unit-move / bounds tests.
test/circuit-editor/dragController.test.mjs	8 shift-extend lifecycle tests.
test/circuit-editor/draggable.test.mjs	4 makeShiftExtendGhost geometry tests.

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Testing

• +26 circuit-editor tests across the 4 test files above, all passing.

• Run locally from source/npm/qsharp/:

  # build the UX bundle
  npm run build:ux
  # run the circuit-editor suite
  node --test "test/circuit-editor/**/*.test.mjs"