new(google/open-vcdiff): VCDIFF (RFC 3284) encoder/decoder

[tannevaled]

Summary

Adds Google's canonical C++ implementation of the VCDIFF (RFC 3284) generic-differencing format.

Why now

VCDIFF underlies several systems still in use:

• Chrome's SDCH (Shared Dictionary Compression over HTTP)
• Google Update protocol (Chromium, Chrome OS)
• Various delta-update pipelines (e.g. some game patchers, embedded firmware deltas)

The upstream repo was archived on 2023-08-26, but the implementation remains RFC-compliant and is the canonical C++ reference. Nothing newer replaces it.

Build

• Autotools (autoreconf -fi + ./configure + make); Apache-2.0.
• Source tarball doesn't include the vendored gtest submodule, but the install path doesn't require it (tests only).

Test plan

Round-trip encode/decode of a small dictionary+target pair, verifying byte-identity after decode. Exercises both the encoder and decoder.

🤖 Generated with Claude Code

[tannevaled]

Parking this PR — source build needs vendored gflags submodule + custom m4 macros.

After 3 iterations, the structural issue is that open-vcdiff's configure.ac uses Google-custom autoconf macros (AC_CXX_STL_NAMESPACE, AC_DEFINE_GOOGLE_NAMESPACE) that live in the gflags/m4/ git submodule, which is not in the GitHub archive tarball.

To make this work properly, the recipe would need to:

1. git clone https://github.com/gflags/gflags.git (at a specific submodule sha — not preserved in the tarball)
2. Optionally clone gtest too (only for unit tests, not the binary)
3. Then autoreconf -fi with -I gflags/m4

Or patch configure.ac to drop the legacy Google macros (AC_CXX_STL_NAMESPACE is obsolete — all C++ now uses std:: namespace).

Since this is an archived upstream (last commit 2023-08-26) and the recipe is a 30-line autotools build, the maintenance ROI is low. Leaving the PR open for review — happy to invest further iterations if maintainers consider it worth it, otherwise closing in favor of a leaner fix later.

Symptoms documented in the recipe / commit log:

• Iter 1 (`d5394f6c`): `not-found: version` (default github releases endpoint hit empty — 0 releases, 5 tags)
• Iter 2 (`e0d4f1af`/`2051bb63`): switched to `/tags` + ignored 2-part `open-vcdiff-0.8` → version resolution worked
• Iter 3 (`f5f7971a`): `mkdir -p gflags/m4 gtest` to satisfy autoreconf's directory probe → uncovered the next layer: m4 macros themselves are missing, not just the dir