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

DESIGN.md

@@ -7,8 +7,8 @@ Work is tracked in Gitea issues; the tracking epic links every piece in dependen
 ## Decisions
 
 - **Placement model:** freeform plops (not a square-foot grid), scaled by real plant spacing. Grid snapping may come later as a toggle.
-- **A fill is one of two operations (#77).** A plop is a *clump*, not a plant, which is the right primitive for SKETCHING ("a few plops of garlic in a corner") but can't draw a real planting — a filled bed comes out as ~15 blobs, not 8 rows of garlic. So `FillRegion`/`FillNamedRegion` take a `FillLayout`: `clump` (default; plop radius 1.5×spacing, ~7 plants each — quick coverage) or `grid` (radius spacing/2, pitch = spacing, ONE plant per plop — a layout you could plant from). Same `hexCenters` lattice and #75 edge rule for both; only the radius→spacing relationship differs. Surfaced on `POST /objects/:id/fill` (`layout`) and the agent's `fill_region` (`mode`). A grid-filled bed approaches the low-hundreds-of-plops the SVG budget was sized for, which the semantic-zoom tiers already anticipate.
-- **Spacing is a plant-to-plant rule, so bed edges get half of it.** A bed edge is not a competitor for soil, light or water, so the outer row owes it half the spacing rather than a full one. `FillRegion` centres its lattice accordingly, and lets a plop — a *clump* three spacings across — cross the edge by up to half a spacing so its outermost plants land at that half-spacing. The rule, the square-foot-chart arithmetic behind it, and the failure mode it prevents are written out once in `hexCenters`; #75 is what getting it wrong looked like.
+- **A fill is one of two operations (#77).** A plop is a *clump*, not a plant, which is the right primitive for SKETCHING ("a few plops of garlic in a corner") but can't draw a real planting — a filled bed comes out as ~15 blobs, not 8 rows of garlic. So `FillRegion`/`FillNamedRegion` take a `FillLayout`: `clump` (default; plop radius 1.5×spacing, ~7 plants each — quick coverage) or `grid` (radius spacing/2, pitch = spacing, ONE plant per plop — a layout you could plant from). Same `hexCenters` lattice and #75 edge rule for both; only the radius→spacing relationship differs. Surfaced on `POST /objects/:id/fill` (`layout`) and the agent's `fill_region` (`mode`). Same centered `hexCenters` lattice for both, but BOTH the plop radius (`plopRadiusFor`) and the edge inset (`edgeInset`) differ by layout: a grid plant sits at the plop's centre, so it insets a half-spacing; a clump's plants reach its rim, so it insets radius-less-a-half and overhangs the edge by that half — reusing the clump formula for grid would inset by zero and plant flush on the edge. A grid-filled bed approaches the low-hundreds-of-plops the SVG budget was sized for, which the semantic-zoom tiers already anticipate.
+- **Spacing is a plant-to-plant rule, so bed edges get half of it.** A bed edge is not a competitor for soil, light or water, so the outer row owes it half the spacing rather than a full one. `FillRegion` centres its lattice accordingly, and lets a plop — a *clump* three spacings across — cross the edge by up to half a spacing so its outermost plants land at that half-spacing. The rule, the square-foot-chart arithmetic behind it, and how it differs by layout are written out once in `edgeInset` (which `hexCenters` then honours); #75 is what getting it wrong looked like.
 - **Stack:** Go 1.26.x backend, module `gitea.stevedudenhoeffer.com/steve/pansy`; React + TypeScript + Vite + Tailwind frontend, production build embedded via `embed.FS` → one static binary (`CGO_ENABLED=0`).
 - **Users:** multi-user with ownership. Users own gardens; a garden can be shared with other users as viewer (read) or editor (edit content). Owner additionally shares/deletes. The first registered user is `is_admin` (set race-free inside the INSERT); admin gates instance-wide Settings (`requireAdmin`), the only thing that reads that flag.
 - **Auth:** OIDC-first (Authentik is the primary IdP), local argon2id passwords as an optional fallback.