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

internal/api/ops_test.go

@@ -61,7 +61,7 @@ func countChangeSets(t *testing.T, r *gin.Engine, cookie *http.Cookie, gardenID
 func TestFillAndClearAPI(t *testing.T) {
 	r := authEngine(t, localCfg())
 	cookie := registerAndCookie(t, r, "[email protected]")
-	gid, objID, plantID := seedFillableBed(t, r, cookie, 200, 200, 20)
+	_, objID, plantID := seedFillableBed(t, r, cookie, 200, 200, 20)
 
 	// Fill by compass name.
 	w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
@@ -96,7 +96,6 @@ func TestFillAndClearAPI(t *testing.T) {
 	if n := int(decodeMap(t, w.Body.Bytes())["cleared"].(float64)); n != 0 {
 		t.Errorf("second clear removed %d, want 0", n)
 	}
-	_ = gid
 }
 
 // TestFillRegionSelectionAPI: exactly one of region/rect, and a rect fills only
@@ -125,6 +124,20 @@ func TestFillRegionSelectionAPI(t *testing.T) {
 		t.Errorf("bad region name = %d, want 400", w.Code)
 	}
 
+	// A zero-area rect is malformed input, not "plant one at the centre". An empty
+	// `"rect": {}` decodes to all-zeros and must be caught the same way.
+	for _, rect := range []map[string]any{
+		{}, // {} → 0,0,0,0
+		{"minXCm": 10, "minYCm": 10, "maxXCm": 10, "maxYCm": 50}, // zero width
+		{"minXCm": 10, "minYCm": 50, "maxXCm": 50, "maxYCm": 50}, // zero height
+		{"minXCm": 50, "minYCm": 50, "maxXCm": 10, "maxYCm": 10}, // inverted
+	} {
+		if w := doJSON(t, r, http.MethodPost, fillPath(objID),
+			map[string]any{"plantId": plantID, "rect": rect}, cookie); w.Code != http.StatusBadRequest {
+			t.Errorf("degenerate rect %v = %d, want 400", rect, w.Code)
+		}
+	}
+
 	// A rect confined to the NE corner produces plops only there. Local frame:
 	// +x east, -y north.
 	w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{