Prep for streaming large media uploads from disk instead of buffering the
whole file in RAM. Add a `ChunkSource` abstraction — `BufferChunkSource`
(in-memory, byte-for-byte the old path) and `FileChunkSource` (disk-backed,
reads exact-length chunks via a FileHandle, loops on short reads, throws on
unexpected EOF) — and change `uploadHighwayHttp` to consume a `ChunkSource`
instead of a `Uint8Array`. The uploader now owns the source and closes it once
in `finally` (best-effort; never masks a primary upload error).
Pure structural refactor: no streaming is wired yet. All five call sites
(pipeline runPuts, group + private file upload, personal + group avatar) wrap
their bytes in `BufferChunkSource`, so behavior is unchanged. Adds a
byte-oracle test proving FileChunkSource ≡ BufferChunkSource across block
boundaries, plus short-read/EOF/empty-input coverage.
DrSmoothl