Add the terrain store: region grids and tile cache

[mouftz]

Summary

Implements the Terrain Store (#31) — all six sub-tickets in one PR, since
the two halves are small and share a module.

• RegionGrids — scratch space for one region. Holds a sum grid and a
  weight grid, accumulates each window as value × weight and weight,
  and divides one by the other in heights(). Written to only by the
  sampler (Clean every window and add the results into the region grids #35).
• TileCache — finished tiles by seed, LRU-evicted at a fixed
  capacity. Touched only by generation orchestration (Generation Orchestration #36).

Adds numpy (2.5.2) as the project's first runtime dependency, per
CONTRIBUTING: pyproject.toml and uv.lock are both committed.

Validation

• scripts/quality-check.sh passes locally
• Appropriate /test commands were run and are passing (e.g. /test gpu)
• Unit tests were added / e2e tests were added where applicable
• Manual testing, if applicable (describe further):

27 tests in tests/test_store.py, covering every assertion listed in
#40–#45. The overlap test uses the worked 1×6 example from #31 directly.
Full suite is 68 passed.

Checklist

• The change is focused and easy to review.
• Tests were added or updated for behaviour changes.
• No secrets, generated files, or files that only make sense on my machine are included.
• Relevant documentation is updated.

Additional Comments

Two decisions worth a second opinion:

• put copies the incoming array so the generator can reuse its buffer,
  but get hands back the cache's own grid rather than a copy — copying
  on every read would undo the point of caching. Callers should treat a
  returned tile as read-only. Happy to change this if you would rather it
  be defensive on both sides.
• Disk persistence is mentioned in the module docstring but has no
  sub-ticket, so it is not implemented here.

Closes #40, #41, #42, #43, #44, #45

[coveralls]

Coverage Report for CI Build 32647543760

Coverage increased (+1.0%) to 97.744%

Details

• Coverage increased (+1.0%) from the base build.
• Patch coverage: 1 uncovered change across 1 file (191 of 192 lines covered, 99.48%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/terrain_diffusion/store.py	56	55	98.21%
Total (2 files)	192	191	99.48%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	532
Covered Lines:	520
Line Coverage:	97.74%
Coverage Strength:	0.98 hits per line

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💛 - Coveralls

[KurbyDoo]

I changed some of the specifications of the original tickets since they don't really match what I'm thinking anymore, could you update this PR accordingly? Thanks

[KurbyDoo]

sorry I'm looking back at the original paper and it doesn't look like there are two different objects to store region grids and tile cache, can we merge the two into one. the only thing that should be cached is the weight value product and the weights as two seperate grids

[KurbyDoo]

I also think caching by seed is no longer best, can we instead cache by tile x, y coordinates. you can assume that every tile will live at a location at some x and y in a global coordinate space. in general tiles divide the global space into a partition with no overlap