Add Lance annotation writer stage

[VibhuJawa]

Split from #2106. This is PR 3 of 3 in the Lance IO stack.

Stacked on #2112. The branch is based on feat/lance-writer; until #2111 and #2112 merge, GitHub may show lower-stack changes in this PR when viewed against main. For the exact PR3 delta, compare:
VibhuJawa/NeMo-Curator@feat/lance-writer...feat/lance-annotation-writer

What changed:

• Adds LanceAnnotationWriter for sparse updates to existing Lance rows using LanceReader metadata columns.
• Adds prepare() to create/validate annotation columns and pin the read version.
• Adds commit_lance_annotation_checkpoint for publishing updated fragments.
• Adds tests for sparse annotation updates, duplicate row rejection, and split-fragment checkpoint rejection.

Validation:

• uv run --extra lance --group test pytest -q tests/stages/text/io/reader/test_lance.py tests/stages/text/io/writer/test_lance.py
• uv run ruff check nemo_curator/stages/text/io/writer/init.py nemo_curator/stages/text/io/writer/lance.py tests/stages/text/io/writer/test_lance.py
• git diff --check

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