fix: preserve paged all-null projections

[QuakeWang]

Problem

Paged buckets store ALL_NULL columns as directory entries with slot_size = 0 and no slot payload. The reader correctly skipped round 2 IO for those slots, but partial projection also used the presence of round 2 ranges to decide whether to construct paged bucket readers.

As a result, when a projection requested only all-null columns from a partial paged bucket, the requested column could be dropped from the output.

Fix

Separate the two concepts:

• slot_size > 0 decides whether a projected column needs round 2 IO.
• projected[gi] decides whether a column needs to appear in the result.

The reader now tracks partial paged buckets explicitly and constructs their readers even when no round 2 IO is required. ALL_NULL projected slots are materialized with an ENCODING_ALL_NULL reader.

Tests

• Added a regression test for projecting only an all-null paged column.
• Ran cargo test --workspace.
• Ran cargo clippy --workspace --all-targets -- -D warnings.

[JingsongLi]

+1