Fill: honour the half-spacing edge rule when packing plops (#75)

Filling a bed left the outer row too far from the edge, and staggered rows
worse still. Two defects, both from anchoring the lattice at the region's min
corner:

- Odd rows offset by `radius` started at `MinX + 2·radius`, leaving a bare
  strip a whole plop wide down one side of every other row.
- All the leftover slack piled up on the far edge, where plops hung 13cm
  outside the bed on a 4×8ft garlic bed. Nothing clips them, so they drew
  over the bed outline.

Spacing is a constraint between neighbouring plants competing for the same
soil, light and water. A bed edge is not a competitor, so the outer row owes
it half the spacing — the arithmetic inside every square-foot-gardening chart
(4/square = 6" apart, 3" from the square's edge).

The wrinkle: a plop is a CLUMP, not a plant. defaultPlopRadius is 1.5×spacing,
so keeping the whole circle inside the bed insets the outer row by 1.5
spacings, three times what the rule allows. So centre the lattice and set the
minimum centre-inset to `radius - spacing/2`: the clump may cross the edge by
up to half a spacing, putting its outermost plants exactly the half-spacing
from the edge the rule asks for. Capped there — a clump mostly outside the bed
would be a drawing of plants in the path.

Same bed, same 15 plops, now symmetric with a deliberate 6.5cm overhang inside
the 7.5cm budget instead of an accidental 13cm on one side only. The stagger
falls out of the centring for free: an offset row holds one fewer plop, and
centring that run puts it exactly half a pitch off its neighbours.

TestFillRegionDeterministicPacking expected 4 plops in a 60×60 bed; the fourth
was centred ON the east edge with half of it outside, well past the budget.
It is 3 now — the fix working, not a regression in it.

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

CLAUDE.md

@@ -85,6 +85,12 @@ Frontend: React 19 + Vite + Tailwind 4 + TanStack Router/Query, built into
   deliberately. `ErrForbidden` means "you can see it but may not do that".
 - **Plops (plantings) live in their parent object's local frame**, origin at the
   object's center, `-y` is north. Moving or rotating a bed moves its plants free.
+- **A plop is a clump, not a plant.** `defaultPlopRadius` is `1.5 × spacing`, so a
+  plop is three spacings across and holds `π·r²/spacing²` plants. Reasoning about
+  fills as if one plop were one plant gets the geometry wrong every time — which
+  is how #75 happened: requiring the whole circle inside the bed inset the outer
+  row by 1.5 spacings when the horticultural rule is *half* a spacing. Spacing is
+  a constraint between neighbouring plants; a bed edge is nobody's neighbour.
 - **Soft removal**: "clear bed" sets `removed_at`; the editor reads
   `removed_at IS NULL`. Hard delete is a different operation.
 - **Migrations** are numbered `.sql` files in `internal/store/migrations/`, run