requests.h2: split DATA chunks when peer dribbles WINDOW_UPDATEs
Client used to wait for a full max_frame credit refill before each
DATA frame. Peers that grant smaller increments (e.g. Scaleway S3's
32712 vs our 32768) stalled at the 56-byte tail per cycle. Now
waits for a 4 KiB minimum then emits whatever credit is available,
splitting a chunk into multiple frames if needed. Add interop
regression (spec: dribbled_windows). Bottler shows upload bar in KiB.
ocaml-crypto ec/rng dunes: lib/c (src/c was renamed).