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

DESIGN.md

@@ -7,6 +7,7 @@ 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.
+- **Spacing is a plant-to-plant rule, so bed edges get half of it.** Spacing describes two plants competing for the same soil, light and water; a bed edge is not a competitor, so the outer row owes it only *half* the spacing — the arithmetic behind every square-foot chart (4/square = 6" apart, 3" from the edge). `FillRegion` centres its lattice and lets a plop, which is a *clump* 3 spacings across, cross the edge by up to half a spacing so its outermost plants land at that half-spacing. See `hexCenters`; #75 is what getting this wrong looked like.
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