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 34997107256

Coverage increased (+0.8%) to 98.593%

Details

• Coverage increased (+0.8%) from the base build.
• Patch coverage: 277 of 277 lines across 2 files are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	782
Covered Lines:	771
Line Coverage:	98.59%
Coverage Strength:	0.99 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]

structure looks good overall!

please remove all comments that are not nessasary, it makes the code hard to read and confusing / misleading for future editors. ideally behaviour should be obvious from variable and function names and comments should only be used for when the reason for doing something is not clear. comments should also not reference issue numbers or talk about things that do not exist in the codebase / are not implemented yet

also add the coverage file to gitignore thanks

[KurbyDoo]

nice looks good thanks!

[KurbyDoo]

lgtm!