Read the store once for the pseudobulk tree; preload when memory allows

[YPARK]

Tree step reads the store once

With the pseudobulk tree on, the reassignment read the whole matrix and then the tree read every top node's cells again, one parallel column read per node. A node's cells are spread over the entire store, so each zarr chunk was decoded once per node, and the concurrent reads thrashed a slow disk.

The matrix is now read once, in column order. The reassignment and the tree share it; each node's block is cut from it in memory (select_columns), which replaces the equivalent test fixture.

Automatic preload

• Loaders preload columns unless LEGUME_AUTO_PRELOAD is 0/false/off.
• The preload budget defaults to half the memory available when the process first asks, shared by every preload of the process. A fixed 8 GiB remains the fallback when the system does not report memory.
• LEGUME_PRELOAD_BUDGET_BYTES still overrides the budget; data over budget stays on the streaming path as before.
• New dependency: sysinfo 0.38 (system feature only; 0.39 needs rustc 1.95).

Other

Cargo.lock is no longer tracked.