Objects are stored once instead of duplicated on every commit, an interrupted write is recoverable instead of corrupting the archive, and a portable incremental exchange bundle format (.hgb/.hgh) ships in hgit-native. format_version stays 4 — nothing here changes what an existing byte means, only what a writer produces and how tolerant a reader is.
See docs/research/11-storage-and-exchange-1.9.md for the full campaign report (measured before/after, test/QEMU evidence, limitations), ADR 0018, ADR 0019, ADR 0020, and BUNDLE.md.
Verified on real TempleOS under QEMU: full regression suite, golden fixtures regenerated, in-guest hgit version confirms HGIT_VERSION 1.9.0.
Install: see INSTALL.md. hgit-bundle-1.9.0.zip is what the Chocolatey package downloads.