perf(encoding): initialize only the page metadata a read will touch

[Ali2Arslan]

Summary

The structural decoder's StructuralFieldScheduler::initialize eagerly initializes every page in a column before scheduling, even when the read only touches one page. For a column with thousands of large, non-coalescing pages (e.g. a BTree leaf scan that reads one leaf), this means paying metadata IO proportional to the page count regardless of how few rows are actually requested.

This PR makes page-metadata initialization range-scoped and per-page:

• DecodeBatchScheduler::try_new and StructuralFieldScheduler::initialize now take an optional requested_ranges: Option<Arc<[Range<u64>]>> — the top-level row ranges that will later be scheduled. The structural path initializes only the pages those ranges overlap (pages_overlapping_ranges). None preserves the prior eager behavior.
• The page-scheduler initialize is split into init_ranges() (declare the byte ranges the page needs) and init_from_buffers(buffers, io) (finish initialization from those bytes). The field scheduler concatenates the misses' ranges into a single submit_request so adjacent ranges coalesce into shared GETs instead of one request per page.
• Page metadata is cached per page via PageDataCacheKey { column_index, page_index, view_tag } (replacing the per-column FieldDataCacheKey). view_tag is retained so a column decoded under two shapes (blob descriptor Struct<pos,size> vs raw LargeBinary) can't collide on cached state.
• Nested schedulers forward ranges consistently with their existing schedule_ranges: lists/maps/structs pass the top-level ranges through unchanged; fixed-size-list scales by dimension. The requested ranges are always a superset of what schedule_ranges/schedule_take later touch, so any page a read schedules was initialized here.

Net effect: a cold point/range read's metadata IO is invariant to the column's total page count.

Test plan

• cargo test -p lance-encoding --lib (393 passed) — the round-trip encoding harness now exercises both the eager path (full scan, None) and the lazy path (range reads and scattered takes pass their requested_ranges).
• New unit test test_initialize_coalesces_missed_page_metadata: an N-page cache miss issues exactly one submit_request carrying every page's metadata range (not one request per page).
• Updated FullZip page-scheduler tests to the init_ranges / init_from_buffers split.
• cargo test -p lance-file --lib (76 passed).
• cargo test -p lance --lib dataset::blob::tests::test_blob_cache_key_distinguishes_views — confirms the per-page key still distinguishes blob decoder views.
• cargo fmt --all and cargo clippy -p lance-encoding -p lance-file --tests --benches -- -D warnings clean.

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