feat: bounded-memory spill-to-tempfile for ReorderBuffer

[oferchen]

Summary

• Adds SpillableReorderBuffer<T> that wraps ReorderBuffer<T> with transparent disk-backed overflow when in-memory footprint exceeds a configurable threshold (default 64 MB)
• Introduces SpillCodec trait for item serialization using a simple length-prefixed binary format, with implementation for DeltaResult
• Adds ReorderBuffer::take() for non-advancing slot extraction, enabling the spill layer to extract items without disturbing delivery order

Design

When successor items accumulate in the reorder buffer due to head-of-line stalls (e.g., a slow delta computation blocks delivery of 100K+ subsequent items), memory grows unboundedly. This PR bounds memory by spilling the highest-sequence items (furthest from delivery) to a SpooledTempFile (in-memory up to 1 MB, then rolls to disk). A hot zone around next_expected is preserved in memory to avoid thrashing. Items are reloaded transparently when their sequence reaches the delivery cursor.

The spill format is a simple [u32 len][payload] record per item, indexed by BTreeMap<sequence, file_offset> for O(log S) reload. Temp files are cleaned up automatically via RAII (Drop).

Test plan

• No spill when under threshold (zero serialization cost)
• Spill triggers when threshold exceeded
• Correct delivery order after spill and reload
• Cleanup on drop (temp files removed)
• Interleaved spill/deliver cycles
• Exact threshold boundary (no spill at boundary, spill above)
• Empty buffer operations
• Force insert with spill
• Spill stats tracking (spill/reload event counters)
• Large-scale spill and drain (100 items)
• DeltaResult SpillCodec roundtrip (Success, NeedsRedo, Failed)
• SpillableReorderBuffer with DeltaResult items
• ReorderBuffer::take() extracts without advancing cursor

Closes #1884