Documents live as canonical BSON bytes in a segmented per-collection slab
(fixed 8 MiB segments keep capacity slack under one segment); the docs map
holds flat offsets that stay valid across segment growth, and removed
documents leave garbage bytes until compaction rewrites. The per-document
ArenaAllocator and its second full Pair-tree copy are gone.
The matcher walks the stored bytes directly, skipping by length any field
the filter does not name (a new bson byte-walker: element_key, skip_value,
read_value with borrowed leaves, get_at, and a borrowed spine parse). The
byte matcher is differential-tested against the tree matcher on a corpus
and shares its operator logic. Stored documents are never materialized on
the scan path or in aggregate $match; $group reads group keys and sums
straight off the bytes. Sort, projection, findAndModify, updates and
index entry generation use a borrowed spine into the slab (or the byte
collector, which also replaced collect_values in build_entries). The
compaction threshold now counts uncompressed data volume, since a
compressed log would otherwise never trigger.
Measured (tests/e2e/results/phase5.txt): server RSS 1979 -> 539 MB (2.4x
smaller than MongoDB; phase1 baseline 2.0 GB), range-scan 22.5 -> ~12 ms
(parity, best run faster than MongoDB), proj 4.1 -> 3.4 ms, createIndex
parity. Verified: unit suite in all three modes with zero leaks, the
crash pair, e2e6, and the stress/spill programs.
