Address architectural issues #1–#7 from codebase review

[veillette]

#1 – Centralize element labels in ElementRegistry
Add a label: TReadOnlyProperty<string> field to ElementDescriptor so the
registry is the single source of truth for element display names. Export
getElementLabel(). Remove the duplicate buildTypeLabels()/TYPE_LABELS
map from EditContainerNode and the now-unused StringManager import there.

#2 – Rename continuousSpectrumSource → ContinuousSpectrumSource
Fix the lone camelCase type key to match the PascalCase convention used by
all other element types. Add a case "continuousSpectrumSource": fall-through
shim in elementSerialization.ts so scenes saved by older versions continue to
load without data loss.

#3 – Replace 4-arg buildEditControls signature with EditControlContext bag
Introduce EditControlContext { signConvention, useCurvatureDisplay } in
EditControlsResult.ts. Update ElementDescriptor, the exported
buildEditControls() functions in ElementRegistry/EditControlFactory, and the
call site in EditContainerNode. Registry entries that previously prefixed
unused params with _ now destructure only what they need from the bag.
Adding a new display preference requires a single interface change, not
touching every builder signature.

#4 – Replace instanceof checks in OpticsScene.simulate() with interfaces
Add IAcquirable and ICompound to OpticsTypes.ts, with corresponding type
guards (isAcquirable, isCompound). DetectorElement implements IAcquirable;
FiberOpticElement implements ICompound. OpticsScene.simulate() uses the
guards instead of instanceof DetectorElement / instanceof FiberOpticElement,
removing its dependency on those concrete classes.

#5 – Make OpticsScene.getElement() O(1)
Add a _elementById: Map<string, OpticalElement> maintained by addElement /
removeElement / clearElements. getElement() is now a single Map lookup
instead of a full array scan.

#6 – Fix SpatialIndex.getAllElements() O(n²) dedup
Replace the all.includes(el) linear scan inside a loop with a
Set<OpticalElement> accumulator, reducing the fast-path dedup from O(n²)
to O(n).

#7 – Extract syncBidirectional helper in SimScreenView
Replace three near-identical let blockXSync = false bidirectional-sync
patterns with a single generic syncBidirectional(propA, propB) helper.
The helper also writes propA's initial value to propB, removing the
separate initialization lines that preceded two of the three blocks.

https://claude.ai/code/session_01H65WqMzMK88Jkra4gAjhE7