FRC: Incrementally verifiable CommP downloads with recovery

[rvagg]

Coming out of filecoin-project/curio#864 from @magik6k, capturing here so we actually do this when time permits.

1. SP exposes endpoint, e.g. GET /piece/{pieceCID}?tree={layer}, returning intermediate CommP tree nodes at a specified granularity (e.g. leaf32m = each node covers 32 MiB of data)
2. Client (another SP, SDK, FilBeam, etc.):
   a. Verifies the returned layer hashes up to the known PieceCID root (client can now trust SP's layer data)
   b. Stream downloads the piece in layer-sized chunks, verifying each chunk's subtree against the known intermediate node
   c. On verification failure, discards only that chunk and retries from that offset (potential recovery)
3. Benefits: fail-fast on bad bits, resume-friendly downloads, no wasted bandwidth on corrupted transfers

Existing implementation of this idea: https://github.com/CIDgravity/gw/blob/main/ributil/repair_car.go (multi-layer recovery from bitflips in CAR streams)

• Go implementations exist for tree operations; need JS/TS equivalents
• Curio already builds these trees internally during CommP calculation
• The idea of saving and using intermediate trees has been discussed for some time, including as part of the CommPact proposal so there is code in various places for this