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.go

@@ -51,6 +51,10 @@ type objectFillRequest struct {
 	// SpacingOverrideCM plants tighter or looser than the plant's mature spacing
 	// without editing the catalog entry.
 	SpacingOverrideCM *float64 `json:"spacingOverrideCm"`
+	// Layout is "clump" (default; fat clumps for a quick sketch) or "grid"
+	// (individual plants in rows at true spacing). Empty = clump. An unknown value
+	// is refused by the service (#77).
+	Layout string `json:"layout"`
 }
 
 func (h *handlers) fillObject(c *gin.Context) {
@@ -84,9 +88,9 @@ func (h *handlers) fillObject(c *gin.Context) {
 			return
 		}
 		region := service.Region{MinX: rect.MinX, MinY: rect.MinY, MaxX: rect.MaxX, MaxY: rect.MaxY}
-		created, err = h.svc.FillRegion(c.Request.Context(), actor, id, region, req.PlantID, req.SpacingOverrideCM)
+		created, err = h.svc.FillRegion(c.Request.Context(), actor, id, region, req.PlantID, req.SpacingOverrideCM, service.FillLayout(req.Layout))
 	} else {
-		created, err = h.svc.FillNamedRegion(c.Request.Context(), actor, id, req.Region, req.PlantID, req.SpacingOverrideCM)
+		created, err = h.svc.FillNamedRegion(c.Request.Context(), actor, id, req.Region, req.PlantID, req.SpacingOverrideCM, service.FillLayout(req.Layout))
 	}
 	if err != nil {
 		writeServiceError(c, err)