docs(gix): document release setup and native conversion gaps

Add installation, validation and coverage guidance for the official
`GixPython==0.1.0` release, using existing interpreters and separate CLI/native
environments. Record upstream requirements with concrete native/Git evidence.

Distinguish available APIs from remaining compatibility gaps. Native config
identifies reference storage and reftable `HEAD` reads report unsupported
storage, but complete repository-extension validation still requires Git.
Gix already exposes linked worktrees of bare main repositories; its documented
`is_bare()` is a configuration flag, not absence of a worktree. Record the
actual dangling-`commondir` and flattened-error issues instead.

Document the missing cheap independent repository clone for per-command
native identity overrides. Ref edits already accept explicit `LogChange`
messages; the missing operation is native Git-compatible whitespace cleanup,
alongside separate HEAD/reflog transaction differences. Keep those fallbacks
and the prohibition on independent Python implementations of Git semantics.

Preserve historical wheel validation, full-suite measurements and future
fixture/state optimization notes with their original revisions and scope.
They are historical observations, not validation of later rewritten trees.

Validation: reviewed the supplied Git/GixPython sources and direct official
GixPython 0.1.0 probes, checked the ledger against the passing native/CLI
regressions, and ran `git diff --check`. This documentation amendment changes
CLI launches by zero.

Author: codex

README.md

@@ -17,7 +17,10 @@ probably the skills to scratch that itch of mine: implement `git` in a way that
 If you like the idea and want to learn more, please head over to [gitoxide](https://github.com/Byron/gitoxide), an
 implementation of 'git' in [Rust](https://www.rust-lang.org).
 
-*(Please note that `gitoxide` is not currently available for use in Python, and that Rust is required.)*
+The experimental `GitPython[gix]` extra uses the local GixPython bindings for
+supported operations, with automatic Git CLI fallback. See the
+[local installation, test commands, and conversion ledger](doc/gix-backend.md).
+GixPython is not yet available on PyPI.
 
 ## GitPython