Support consumer-selected bundle store paths

[iskandr]

Summary

Closes #93. Stacked on #97; this PR's diff is only the store-path callback change. Merge #97 first and retarget this PR to master before landing. Both changes are prepared in the prospective 1.17.0 release; neither PR is merged or published.

• Add store_path(name, version) to VersionedDatasetRegistry, consistent with the archive registry.
• Require exactly one destination strategy: cache_root, existing callable cache_dir, or the exact-path callback.
• Construction does not invoke callbacks; custom path resolution computes a Path without filesystem inspection, directory creation, locks or downloads.
• All existing inspect/install/refresh/recovery methods use the chosen managed store; internal immutable generations and foreign-directory protection are unchanged.
• Document migration into new managed source directories beside application-owned legacy data/indexes.

Verification

• 845 tests pass; 93% coverage.
• 95 focused registry/bundle/API-reference tests pass.
• Both offline examples, Ruff, and git diff --check pass.
• Regressions cover pure default/sibling-version resolution, custom layout installation and refresh, read-only offline reuse, interrupted-publication recovery, foreign directories/symlinks, strategy exclusivity, and existing root APIs.

No PyEnsembl code is changed.

[coveralls]

Coverage Report for CI Build 36963646264

Coverage increased (+0.09%) to 93.438%

Details

• Coverage increased (+0.09%) from the base build.
• Patch coverage: 9 of 9 lines across 1 file 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:	2606
Covered Lines:	2435
Line Coverage:	93.44%
Coverage Strength:	3.73 hits per line

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💛 - Coveralls

[coveralls]

Coverage Report for CI Build 36963646264

Coverage increased (+0.09%) to 93.438%

Details

• Coverage increased (+0.09%) from the base build.
• Patch coverage: 9 of 9 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

---

Coverage Stats

Relevant Lines:	2606
Covered Lines:	2435
Line Coverage:	93.44%
Coverage Strength:	3.73 hits per line

---

💛 - Coveralls