Address Gadfly findings on #89

- Reject a degenerate (zero-area or inverted) fill rect with 400 instead of
  silently planting a single plop at the object centre. An empty `"rect": {}`
  decodes to all-zeros and is caught the same way. New fillRect.degenerate()
  and a table test covering {}, zero-width, zero-height, and inverted.
- Make the rect a named type (fillRect) rather than an anonymous inline struct,
  matching the rest of internal/api, and bind the pointer to a local in the
  handler so the deref is visibly guarded rather than reading req.Rect.MinX
  under an invariant from a line above.
- ops_test: drop gid from the unpack instead of the `_ = gid` shim.
- ClearBedModal: drop the `const n = plopCount` no-op alias.

Not taken: moving createPlantAPI/makeFillPlant to a shared helper — that
consolidation spans this branch and #88 and is cleanest once both land, as
noted in the PR. Reusing an objectPlantingsPath helper: there isn't one in this
package, and the one inline use doesn't earn a new helper on its own.

Co-Authored-By: Claude Opus 4.8 (1M context) <[email protected]>
Claude-Session: https://claude.ai/code/session_01H3zbym8Doka2d7D48maSgZ

web/src/editor/ClearBedModal.tsx

@@ -18,12 +18,12 @@ export function ClearBedModal({
   onClose: () => void
 }) {
   const clear = useClearObject(gardenId)
-  const n = plopCount
   return (
     <Modal title="Clear bed" onClose={onClose} busy={clear.isPending}>
       <div className="flex flex-col gap-4">
         <p className="text-sm text-muted">
-          Remove all <span className="font-medium text-fg">{n}</span> {n === 1 ? 'plant' : 'plants'} from{' '}
+          Remove all <span className="font-medium text-fg">{plopCount}</span>{' '}
+          {plopCount === 1 ? 'plant' : 'plants'} from{' '}
           <span className="font-medium text-fg">{objectName}</span>? They're marked removed but kept in history.
         </p>
         <div className="flex justify-end gap-2">
@@ -33,7 +33,7 @@ export function ClearBedModal({
           <Button
             type="button"
             variant="danger"
-            disabled={clear.isPending || n === 0}
+            disabled={clear.isPending || plopCount === 0}
             onClick={() => clear.mutate(objectId, { onSuccess: onClose })}
           >
             {clear.isPending ? 'Clearing…' : 'Clear bed'}