Add origin offset to attenuate_all for translation-invariant bucketing

[amarcozzi]

Closes #48.

What

Adds an origin=(ox, oy) parameter to attenuate_all so the shadow-bucketing
plane sweep is anchored to a shared global frame, making the algorithm
translation-invariant.

Why

The sweep floors rotated coordinates onto a 1×1 lattice
(plane_sweep, general.py:27/:33).
floor is a phase-dependent quantizer, so the same physical canopy produces
different irradiance depending on where its coordinates start. A sub-window of a
larger domain therefore cannot reproduce a single whole-domain run — which
blocks tiling a large domain into parallel per-tile runs.

How

The offset is added to the leaf-area, terrain, and sensor coordinates before
rotation only. Because the rotation is linear, a constant world offset before
rotating shifts every rotated coordinate consistently, so two sub-windows of the
same global domain floor into the same lattice. Outputs stay in the input's
local frame — only the shadow grouping is re-anchored. To tile: give each window
the offset of its lower-left corner in the global frame.

Default origin=(0.0, 0.0) is a no-op that reproduces the un-anchored result
exactly.

Tests (TDD)

Written before the implementation:

• test_origin_default_matches_no_arg — origin=(0, 0) reproduces the no-arg
  result byte-for-byte across canopy, terrain, and sensors.
• test_origin_makes_canopy_translation_invariant — the same canopy expressed at
  a shifted origin gives identical irradiance when origin cancels the shift
  (0% of cells differ), with a guard asserting the shift genuinely perturbs the
  bucketing when not re-anchored (so the test can't pass vacuously).

Full suite: 57 passed. The stored-result assert_equal tests confirm the
default path is unchanged.

Scope

Kept to attenuate_all, the path the tiling consumer (FastFuels treevox) uses.
attenuate_surface is left unchanged. Independent of #49 (voxel_dim
anisotropy), but the two interact: correcting that geometry would change shadow
reach, and hence the up-sun halo a tiling caller supplies.

[allisonrenie]

Looks good to me. Planning to merge this first then get the second PR in