Fill mode: a "grid" layout that plants individual plants in rows (#77)

Steve chose option 3: keep clumps as the default primitive, add a grid/rows
fill mode, so sketching and planning are different operations with different
outputs rather than one model forced to be both.

A plop is a CLUMP, not a plant — great for "a few plops of garlic in a corner",
useless for drawing a plantable 8-rows-of-garlic bed (that came out as ~15
blobs, #77). FillLayout selects what a fill packs:
- clump (default, unchanged): radius 1.5×spacing, ~7 plants per plop.
- grid: radius spacing/2, pitch = spacing, ONE plant per plop — rows you could
  actually plant from.

The geometry is the SAME hexCenters lattice and the SAME #75 half-spacing edge
rule; only the radius→spacing relationship differs (plopRadiusFor). Grid keeps
no 15cm floor — its whole point is true spacing — while clump keeps it so a
tiny-spacing plant doesn't make invisible clumps.

Threaded through FillRegion/FillNamedRegion (empty layout = clump, so existing
callers are unchanged; unknown layout = ErrInvalidInput), the REST /fill
endpoint (`layout`), and the agent's fill_region tool (`mode`, enum clump|grid),
so "plant the bed in rows" works.

Tests: grid produces many more, single-plant plops than clump on the same bed
(radius spacing/2, derived count 1); unknown layout is refused at both the
service and the API. maxFillPlops still caps a grid fill of a huge bed.

No frontend fill affordance exists yet (fill is agent-only in the UI; the fill
UI was deferred in #82), so the mode toggle rides along when that's built —
noted. Docs: DESIGN placement-model decision.

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

internal/api/ops_test.go

@@ -95,6 +95,27 @@ 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.
+	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)
+	}
+	// 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 {
+		t.Errorf("unknown layout = %d, want 400", w.Code)
+	}
 }
 
 // TestFillRegionSelectionAPI: exactly one of region/rect, and a rect fills only