Snapshot summary sizes lose precision: 5-decimal GB strings round small sizes to 0 B #168

The snapshot collector pre-formats every *files-size summary value as gigabytes with 5 decimal places (backend/collectors/collect_snapshots.py:68). One step of 0.00001 GB is ~10.7 KiB, which makes the frontend display provably wrong information on small tables:

1. Real files shown as 0 B. A commit that adds 2 data files with 3 records reports added-files-size: 0.00000 GB — the Timeline details panel renders Files size 0 B for files that exist and hold data.
2. Arithmetic that doesn't reconcile. Table state shows 21 KiB → 10 KiB while the impact line shows −10 KiB (raw values 20.97 and 10.49 KiB each round independently). Users read 21 − 10 = 11 and report a bug.

The frontend already reverses the GB formatting to bytes (frontend/src/features/timeline/lib/format/backendSize.ts), so it can only recover what survived the 5-decimal rounding — any frontend "fix" would be fake precision.

Proposal: ship raw byte counts (Iceberg's summary values are byte strings already, e.g. added-files-size: "5735") and let the frontend format once at display time. Alternatively keep the GB strings but with enough decimals (~9) to be byte-exact. Either way it's a breaking change to the graph payload consumed by the frontend and icegraph-client, so it should ride a version bump.