Address fill-mode review: grid edge inset + drop dead coverage append

Gadfly findings on #95:

- Correctness (3 models): the shared inset formula radius-spacing/2
  collapses to 0 in grid mode (radius = spacing/2), so grid's outer row
  planted flush on / overhanging the bed edge instead of the half-spacing
  in that the rule wants. The inset genuinely differs by layout — a grid
  plant sits AT the plop centre (inset spacing/2), a clump's plants reach
  its rim (inset radius-spacing/2, overhanging by a half). Split it into a
  new edgeInset(radius, spacing, layout); hexCenters now takes a
  precomputed inset and is pure geometry (no spacing/layout knowledge).
  Regression guard: grid plants land at ±25 on a 60cm bed, not ±30.

- Performance (2 findings): the in-loop `existing = append(existing, *p)`
  was dead — every plop in one fill shares a radius and sits on a distinct
  lattice point, and a plop is "covered" only when wholly inside another,
  impossible between equal-radius circles at different centres. Removing it
  stops the coveredByExisting scan growing during the fill (an empty-bed
  grid fill's check was needlessly quadratic in the plop count).

- Docs: FillRegion/fillLoaded/hexCenters comments and the DESIGN.md bullets
  updated for plopRadiusFor/edgeInset (were still citing defaultPlopRadius
  and "written out in hexCenters").

- Test hygiene: split the grid + bad-layout cases out of TestFillAndClearAPI
  into TestFillLayoutAPI (one concern per test).

The enum-tag finding is a non-issue: majordomo's DefineTool derives its arg
schema from the same struct-tag reflection as Generate (proven by the vision
SeedPacket enum), and the service validates mode regardless.

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

internal/api/ops_test.go

@@ -95,22 +95,39 @@ 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)
 	}
+}
 
-	// Grid layout (#77) packs individual plants at true spacing — many more, small
-	// plops than the clump default on the same bed.
+// TestFillLayoutAPI covers the layout selector (#77): grid packs denser than the
+// clump default, and an unknown layout is a 400 rather than a silent clump fill.
+func TestFillLayoutAPI(t *testing.T) {
+	r := authEngine(t, localCfg())
+	cookie := registerAndCookie(t, r, "[email protected]")
+	_, objID, plantID := seedFillableBed(t, r, cookie, 200, 200, 20)
+
+	// Clump (default) for the baseline count.
+	w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
+		"plantId": plantID, "region": "all",
+	}, cookie)
+	if w.Code != http.StatusOK {
+		t.Fatalf("clump fill: status %d, body %s", w.Code, w.Body.String())
+	}
+	clumpN := int(decodeMap(t, w.Body.Bytes())["created"].(float64))
+	if w := doJSON(t, r, http.MethodPost, clearPath(objID), nil, cookie); w.Code != http.StatusOK {
+		t.Fatalf("clear between fills: %d", w.Code)
+	}
+
+	// Grid packs individual plants at true spacing — many more, small plops.
 	w = doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
 		"plantId": plantID, "region": "all", "layout": "grid",
 	}, cookie)
 	if w.Code != http.StatusOK {
 		t.Fatalf("grid fill: status %d, body %s", w.Code, w.Body.String())
 	}
-	if gridN := int(decodeMap(t, w.Body.Bytes())["created"].(float64)); gridN <= created {
-		t.Errorf("grid fill created %d, want more than the clump fill's %d", gridN, created)
+	if gridN := int(decodeMap(t, w.Body.Bytes())["created"].(float64)); gridN <= clumpN {
+		t.Errorf("grid fill created %d, want more than the clump fill's %d", gridN, clumpN)
 	}
+
 	// An unknown layout is a 400, not a silent clump fill.
-	if w := doJSON(t, r, http.MethodPost, clearPath(objID), nil, cookie); w.Code != http.StatusOK {
-		t.Fatalf("clear before bad-layout: %d", w.Code)
-	}
 	if w := doJSON(t, r, http.MethodPost, fillPath(objID), map[string]any{
 		"plantId": plantID, "region": "all", "layout": "spiral",
 	}, cookie); w.Code != http.StatusBadRequest {