Add MSOA coverage and centroid datasets

[fcorowe]

Summary

• add coverage_msoa as packaged lazy data and normalized inst/extdata CSV
• add msoa_centroids as packaged lazy data and normalized inst/extdata CSV
• update data build, metadata, documentation, README, path helper, and data contract tests

Validation

• devtools::test(): 213 passed, 0 failed
• devtools::check(document = FALSE, manual = FALSE, args = "--no-build-vignettes"): 0 errors, 0 warnings, 2 existing-style notes
• git diff --check

Closes #9
Closes #10

Note: #11 remains open because the issue body is empty; README changes here may address it, but that needs confirmation.