fix(core): preserve configured blob payloads

[dcfocus]

Summary

• keep configured context payload columns inline and mark them with neutral schema metadata for lazy default projections
• recover blob-column configuration on reopen and reject legacy blob-v2 schemas with an actionable error
• preserve lazy payloads during partial updates and materialize text when lexical retrieval needs it
• add regressions for binary round trips, WAL merge, reopen, updates, text retrieval, and legacy-schema rejection

Compatibility

Datasets already created with lance-encoding:blob cannot be recovered through MemWAL today: reads return no payload and WAL merge fails. This change fails fast when opening those schemas rather than continuing with silent data loss. New datasets store payloads inline and exclude configured blob columns from default bulk projections.

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace --all-targets
• cd python && uv run --extra tests pytest (213 passed, 6 skipped, 3 xfailed)
• cd python && uv run ruff format --check .
• cd python && uv run ruff check .
• cd python && uv run --extra dev pyright

Closes #222