refactor: move registration of namespaces on kernel add into elaborator

[Kha]

Kernel checking will be moved to a different thread but namespace registration should stay on the elaboration thread

Stacked on #5145

[leanprover-community-bot]

Mathlib CI status (docs):

• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase b862e2d25163584322bcca54e5e56d671da1f258 --onto 48be424eaa2ae06972e9cfec4d355906b532204d. (2024-12-12 16:33:10)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase 48eb3084a0e0e15d6c22287d1c17072cbe9f1541 --onto dd6445515ddc71826c76b8020fe9e030579b432b. (2025-01-09 17:13:29)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c7939cfb03c78820b069ba9c931900d44b11f58e --onto d2c4471cfa4611977bf4927b5cd849df1a4272b7. (2025-01-12 21:50:37)
• ❗ Batteries/Mathlib CI will not be attempted unless your PR branches off the nightly-with-mathlib branch. Try git rebase c7fd8733334bb0053e4e7ce39b4d241abe08f456 --onto a955708b6c5f25e7f9c9ae7b951f8f3d5aefe377. (2025-01-16 15:31:23)