Add splitfdstream crate for binary streaming format

Add a new crate implementing the splitfdstream binary format, which
enables streaming tar archives with external file references.

The format uses an 8-byte LE signed prefix followed by optional data:
- Negative prefix: inline data of size |prefix|
- Non-negative prefix: external file descriptor reference by index

Key types:
- SplitfdstreamWriter: Build splitfdstreams with inline/external chunks
- SplitfdstreamReader: Parse splitfdstreams back into chunks
- SplitfdstreamTarReader: Read adapter that reconstructs byte streams

This enables zero-copy tar reconstruction from containers-storage's
tar-split metadata by streaming headers inline while referencing
file content via fd.

Assisted-by: OpenCode (Opus 4.5)
Signed-off-by: Colin Walters <walters@verbum.org>

Author: cgwalters

crates/splitfdstream/Cargo.toml

@@ -0,0 +1,21 @@
+[package]
+name = "splitfdstream"
+description = "Binary format for serializing data with external file descriptor references"
+keywords = ["splitfdstream", "fd", "serialization"]
+
+edition.workspace = true
+license.workspace = true
+readme.workspace = true
+repository.workspace = true
+rust-version.workspace = true
+version.workspace = true
+
+[dependencies]
+rustix = { version = "1.0.0", default-features = false, features = ["fs", "std"] }
+
+[dev-dependencies]
+proptest = "1"
+tempfile = { version = "3.8.0", default-features = false }
+
+[lints]
+workspace = true